[edk2,1/3] ArmPkg: add driver to force 64-bit MMIO BARs to be allocated above 4 GB

Message ID 1473674479-20207-2-git-send-email-ard.biesheuvel@linaro.org
State New
Headers show

Commit Message

Ard Biesheuvel Sept. 12, 2016, 10:01 a.m.
By default, the generic PciBusDxe driver uses the presence of a ROM BAR
as a hint that all MMIO BARs should be allocated in the 32-bit region,
even the ones that could be allocated above 4 GB. The reason is that
such a ROM BAR may contain code that runs in the context of a legacy
BIOS (i.e., a CSM), and allocating the 64-bit BARs above 4 GB may put
them out of reach.

Of course, none of this matters on ARM, and so we can unconditionally
override this decision. So take the OVMF implementation of the
IncompatiblePciDeviceSupportDxe driver, rip out the bits that care
about the presence of a legacy BIOS, and wire it up to the ArmPkg PCI
PCDs.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>

---
 ArmPkg/Drivers/IncompatiblePciDeviceSupportDxe/IncompatiblePciDeviceSupport.c   | 223 ++++++++++++++++++++
 ArmPkg/Drivers/IncompatiblePciDeviceSupportDxe/IncompatiblePciDeviceSupport.inf |  49 +++++
 2 files changed, 272 insertions(+)

-- 
2.7.4

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Comments

Leif Lindholm Sept. 12, 2016, 10:23 a.m. | #1
On Mon, Sep 12, 2016 at 11:01:17AM +0100, Ard Biesheuvel wrote:
> By default, the generic PciBusDxe driver uses the presence of a ROM BAR

> as a hint that all MMIO BARs should be allocated in the 32-bit region,

> even the ones that could be allocated above 4 GB. The reason is that

> such a ROM BAR may contain code that runs in the context of a legacy

> BIOS (i.e., a CSM), and allocating the 64-bit BARs above 4 GB may put

> them out of reach.

> 

> Of course, none of this matters on ARM, and so we can unconditionally

> override this decision. So take the OVMF implementation of the

> IncompatiblePciDeviceSupportDxe driver, rip out the bits that care

> about the presence of a legacy BIOS, and wire it up to the ArmPkg PCI

> PCDs.


While I agree this solves a real problem, we're now at a point where
we're adding multiple new implementations of a library to give
standards-compliant platforms the ability to circumvent legacy
workarounds in core code.

Should this not be the other way around?

And if not, is that not a policy decision that suggests the workaround
belongs as a core library anyway?

/
    Leif

> Contributed-under: TianoCore Contribution Agreement 1.0

> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>

> ---

>  ArmPkg/Drivers/IncompatiblePciDeviceSupportDxe/IncompatiblePciDeviceSupport.c   | 223 ++++++++++++++++++++

>  ArmPkg/Drivers/IncompatiblePciDeviceSupportDxe/IncompatiblePciDeviceSupport.inf |  49 +++++

>  2 files changed, 272 insertions(+)

> 

> diff --git a/ArmPkg/Drivers/IncompatiblePciDeviceSupportDxe/IncompatiblePciDeviceSupport.c b/ArmPkg/Drivers/IncompatiblePciDeviceSupportDxe/IncompatiblePciDeviceSupport.c

> new file mode 100644

> index 000000000000..3f2869f8f3c2

> --- /dev/null

> +++ b/ArmPkg/Drivers/IncompatiblePciDeviceSupportDxe/IncompatiblePciDeviceSupport.c

> @@ -0,0 +1,223 @@

> +/** @file

> +  A simple DXE_DRIVER that causes the PCI Bus UEFI_DRIVER to allocate 64-bit

> +  MMIO BARs above 4 GB, regardless of option ROM availability, conserving

> +  32-bit MMIO aperture for 32-bit BARs.

> +

> +  Copyright (C) 2016, Red Hat, Inc.

> +  Copyright (C) 2016, Linaro, Ltd.

> +

> +  This program and the accompanying materials are licensed and made available

> +  under the terms and conditions of the BSD License which accompanies this

> +  distribution. The full text of the license may be found at

> +  http://opensource.org/licenses/bsd-license.php

> +

> +  THE PROGRAM IS DISTRIBUTED UNDER THE BSD LICENSE ON AN "AS IS" BASIS, WITHOUT

> +  WARRANTIES OR REPRESENTATIONS OF ANY KIND, EITHER EXPRESS OR IMPLIED.

> +**/

> +

> +#include <IndustryStandard/Acpi10.h>

> +#include <IndustryStandard/Pci22.h>

> +

> +#include <Library/DebugLib.h>

> +#include <Library/MemoryAllocationLib.h>

> +#include <Library/PcdLib.h>

> +#include <Library/UefiBootServicesTableLib.h>

> +

> +#include <Protocol/IncompatiblePciDeviceSupport.h>

> +

> +//

> +// The protocol interface this driver produces.

> +//

> +STATIC

> +EFI_INCOMPATIBLE_PCI_DEVICE_SUPPORT_PROTOCOL    mIncompatiblePciDeviceSupport;

> +

> +//

> +// Configuration template for the CheckDevice() protocol member function.

> +//

> +// Refer to Table 20 "ACPI 2.0 & 3.0 QWORD Address Space Descriptor Usage" in

> +// the Platform Init 1.4a Spec, Volume 5.

> +//

> +// This structure is interpreted by the UpdatePciInfo() function in the edk2

> +// PCI Bus UEFI_DRIVER.

> +//

> +#pragma pack (1)

> +typedef struct {

> +  EFI_ACPI_ADDRESS_SPACE_DESCRIPTOR AddressSpaceDesc;

> +  EFI_ACPI_END_TAG_DESCRIPTOR       EndDesc;

> +} MMIO64_PREFERENCE;

> +#pragma pack ()

> +

> +STATIC CONST MMIO64_PREFERENCE mConfiguration = {

> +  //

> +  // AddressSpaceDesc

> +  //

> +  {

> +    ACPI_ADDRESS_SPACE_DESCRIPTOR,                 // Desc

> +    (UINT16)(                                      // Len

> +      sizeof (EFI_ACPI_ADDRESS_SPACE_DESCRIPTOR) -

> +      OFFSET_OF (

> +        EFI_ACPI_ADDRESS_SPACE_DESCRIPTOR,

> +        ResType

> +        )

> +      ),

> +    ACPI_ADDRESS_SPACE_TYPE_MEM,                   // ResType

> +    PCI_ACPI_UNUSED,                               // GenFlag

> +    PCI_ACPI_UNUSED,                               // SpecificFlag

> +    64,                                            // AddrSpaceGranularity:

> +                                                   //   aperture selection hint

> +                                                   //   for BAR allocation

> +    PCI_ACPI_UNUSED,                               // AddrRangeMin

> +    PCI_BAR_OLD_ALIGN,                             // AddrRangeMax:

> +                                                   //   no special alignment

> +                                                   //   for affected BARs

> +    PCI_BAR_ALL,                                   // AddrTranslationOffset:

> +                                                   //   hint covers all

> +                                                   //   eligible BARs

> +    PCI_BAR_NOCHANGE                               // AddrLen:

> +                                                   //   use probed BAR size

> +  },

> +  //

> +  // EndDesc

> +  //

> +  {

> +    ACPI_END_TAG_DESCRIPTOR,                       // Desc

> +    0                                              // Checksum: to be ignored

> +  }

> +};

> +

> +

> +/**

> +  Returns a list of ACPI resource descriptors that detail the special resource

> +  configuration requirements for an incompatible PCI device.

> +

> +  Prior to bus enumeration, the PCI bus driver will look for the presence of

> +  the EFI_INCOMPATIBLE_PCI_DEVICE_SUPPORT_PROTOCOL. Only one instance of this

> +  protocol can be present in the system. For each PCI device that the PCI bus

> +  driver discovers, the PCI bus driver calls this function with the device's

> +  vendor ID, device ID, revision ID, subsystem vendor ID, and subsystem device

> +  ID. If the VendorId, DeviceId, RevisionId, SubsystemVendorId, or

> +  SubsystemDeviceId value is set to (UINTN)-1, that field will be ignored. The

> +  ID values that are not (UINTN)-1 will be used to identify the current device.

> +

> +  This function will only return EFI_SUCCESS. However, if the device is an

> +  incompatible PCI device, a list of ACPI resource descriptors will be returned

> +  in Configuration. Otherwise, NULL will be returned in Configuration instead.

> +  The PCI bus driver does not need to allocate memory for Configuration.

> +  However, it is the PCI bus driver's responsibility to free it. The PCI bus

> +  driver then can configure this device with the information that is derived

> +  from this list of resource nodes, rather than the result of BAR probing.

> +

> +  Only the following two resource descriptor types from the ACPI Specification

> +  may be used to describe the incompatible PCI device resource requirements:

> +  - QWORD Address Space Descriptor (ACPI 2.0, section 6.4.3.5.1; also ACPI 3.0)

> +  - End Tag (ACPI 2.0, section 6.4.2.8; also ACPI 3.0)

> +

> +  The QWORD Address Space Descriptor can describe memory, I/O, and bus number

> +  ranges for dynamic or fixed resources. The configuration of a PCI root bridge

> +  is described with one or more QWORD Address Space Descriptors, followed by an

> +  End Tag. See the ACPI Specification for details on the field values.

> +

> +  @param[in]  This                Pointer to the

> +                                  EFI_INCOMPATIBLE_PCI_DEVICE_SUPPORT_PROTOCOL

> +                                  instance.

> +

> +  @param[in]  VendorId            A unique ID to identify the manufacturer of

> +                                  the PCI device.  See the Conventional PCI

> +                                  Specification 3.0 for details.

> +

> +  @param[in]  DeviceId            A unique ID to identify the particular PCI

> +                                  device. See the Conventional PCI

> +                                  Specification 3.0 for details.

> +

> +  @param[in]  RevisionId          A PCI device-specific revision identifier.

> +                                  See the Conventional PCI Specification 3.0

> +                                  for details.

> +

> +  @param[in]  SubsystemVendorId   Specifies the subsystem vendor ID. See the

> +                                  Conventional PCI Specification 3.0 for

> +                                  details.

> +

> +  @param[in]  SubsystemDeviceId   Specifies the subsystem device ID. See the

> +                                  Conventional PCI Specification 3.0 for

> +                                  details.

> +

> +  @param[out] Configuration       A list of ACPI resource descriptors that

> +                                  detail the configuration requirement.

> +

> +  @retval EFI_SUCCESS   The function always returns EFI_SUCCESS.

> +**/

> +STATIC

> +EFI_STATUS

> +EFIAPI

> +CheckDevice (

> +  IN  EFI_INCOMPATIBLE_PCI_DEVICE_SUPPORT_PROTOCOL  *This,

> +  IN  UINTN                                         VendorId,

> +  IN  UINTN                                         DeviceId,

> +  IN  UINTN                                         RevisionId,

> +  IN  UINTN                                         SubsystemVendorId,

> +  IN  UINTN                                         SubsystemDeviceId,

> +  OUT VOID                                          **Configuration

> +  )

> +{

> +  //

> +  // Unlike the general description of this protocol member suggests, there is

> +  // nothing incompatible about the PCI devices that we'll match here. We'll

> +  // match all PCI devices, and generate exactly one QWORD Address Space

> +  // Descriptor for each. That descriptor will instruct the PCI Bus UEFI_DRIVER

> +  // not to degrade 64-bit MMIO BARs for the device, even if a PCI option ROM

> +  // BAR is present on the device.

> +  //

> +

> +  //

> +  // This member function is mis-specified actually: it is supposed to allocate

> +  // memory, but as specified, it could not return an error status. Thankfully,

> +  // the edk2 PCI Bus UEFI_DRIVER actually handles error codes; see the

> +  // UpdatePciInfo() function.

> +  //

> +  *Configuration = AllocateCopyPool (sizeof mConfiguration, &mConfiguration);

> +  if (*Configuration == NULL) {

> +    DEBUG ((EFI_D_WARN,

> +      "%a: 64-bit MMIO BARs may be degraded for PCI 0x%04x:0x%04x (rev %d)\n",

> +      __FUNCTION__, (UINT32)VendorId, (UINT32)DeviceId, (UINT8)RevisionId));

> +    return EFI_OUT_OF_RESOURCES;

> +  }

> +  return EFI_SUCCESS;

> +}

> +

> +

> +/**

> +  Entry point for this driver.

> +

> +  @param[in] ImageHandle  Image handle of this driver.

> +  @param[in] SystemTable  Pointer to SystemTable.

> +

> +  @retval EFI_SUCESS       Driver has loaded successfully.

> +  @retval EFI_UNSUPPORTED  PCI resource allocation has been disabled.

> +  @retval EFI_UNSUPPORTED  There is no 64-bit PCI MMIO aperture.

> +  @return                  Error codes from lower level functions.

> +

> +**/

> +EFI_STATUS

> +EFIAPI

> +DriverInitialize (

> +  IN EFI_HANDLE       ImageHandle,

> +  IN EFI_SYSTEM_TABLE *SystemTable

> +  )

> +{

> +  //

> +  // If the PCI Bus driver is not supposed to allocate resources, then it makes

> +  // no sense to install a protocol that influences the resource allocation.

> +  //

> +  // Similarly, if there is no 64-bit PCI MMIO aperture, then 64-bit MMIO BARs

> +  // have to be allocated under 4 GB unconditionally.

> +  //

> +  if (PcdGetBool (PcdPciDisableBusEnumeration) ||

> +      PcdGet64 (PcdPciMmio64Size) == 0) {

> +    return EFI_UNSUPPORTED;

> +  }

> +

> +  mIncompatiblePciDeviceSupport.CheckDevice = CheckDevice;

> +  return gBS->InstallMultipleProtocolInterfaces (&ImageHandle,

> +                &gEfiIncompatiblePciDeviceSupportProtocolGuid,

> +                &mIncompatiblePciDeviceSupport, NULL);

> +}

> diff --git a/ArmPkg/Drivers/IncompatiblePciDeviceSupportDxe/IncompatiblePciDeviceSupport.inf b/ArmPkg/Drivers/IncompatiblePciDeviceSupportDxe/IncompatiblePciDeviceSupport.inf

> new file mode 100644

> index 000000000000..0bda20199d51

> --- /dev/null

> +++ b/ArmPkg/Drivers/IncompatiblePciDeviceSupportDxe/IncompatiblePciDeviceSupport.inf

> @@ -0,0 +1,49 @@

> +## @file

> +# A simple DXE_DRIVER that causes the PCI Bus UEFI_DRIVER to allocate 64-bit

> +# MMIO BARs above 4 GB, regardless of option ROM availability, conserving

> +# 32-bit MMIO aperture for 32-bit BARs.

> +#

> +# Copyright (C) 2016, Red Hat, Inc.

> +# Copyright (C) 2016, Linaro, Ltd.

> +#

> +# This program and the accompanying materials are licensed and made available

> +# under the terms and conditions of the BSD License which accompanies this

> +# distribution. The full text of the license may be found at

> +# http://opensource.org/licenses/bsd-license.php

> +#

> +# THE PROGRAM IS DISTRIBUTED UNDER THE BSD LICENSE ON AN "AS IS" BASIS, WITHOUT

> +# WARRANTIES OR REPRESENTATIONS OF ANY KIND, EITHER EXPRESS OR IMPLIED.

> +##

> +

> +[Defines]

> +  INF_VERSION                    = 0x00010005

> +  BASE_NAME                      = IncompatiblePciDeviceSupportDxe

> +  FILE_GUID                      = 18BCB238-844E-446B-AA2D-3DF38A25FA0F

> +  MODULE_TYPE                    = DXE_DRIVER

> +  VERSION_STRING                 = 1.0

> +  ENTRY_POINT                    = DriverInitialize

> +

> +[Sources]

> +  IncompatiblePciDeviceSupport.c

> +

> +[Packages]

> +  ArmPkg/ArmPkg.dec

> +  MdeModulePkg/MdeModulePkg.dec

> +  MdePkg/MdePkg.dec

> +

> +[LibraryClasses]

> +  DebugLib

> +  MemoryAllocationLib

> +  PcdLib

> +  UefiBootServicesTableLib

> +  UefiDriverEntryPoint

> +

> +[Protocols]

> +  gEfiIncompatiblePciDeviceSupportProtocolGuid               ## PRODUCES

> +

> +[Pcd]

> +  gArmTokenSpaceGuid.PcdPciMmio64Size                        ## CONSUMES

> +  gEfiMdeModulePkgTokenSpaceGuid.PcdPciDisableBusEnumeration ## CONSUMES

> +

> +[Depex]

> +  TRUE

> -- 

> 2.7.4

> 

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Laszlo Ersek Sept. 12, 2016, 12:29 p.m. | #2
On 09/12/16 12:23, Leif Lindholm wrote:
> On Mon, Sep 12, 2016 at 11:01:17AM +0100, Ard Biesheuvel wrote:

>> By default, the generic PciBusDxe driver uses the presence of a ROM BAR

>> as a hint that all MMIO BARs should be allocated in the 32-bit region,

>> even the ones that could be allocated above 4 GB. The reason is that

>> such a ROM BAR may contain code that runs in the context of a legacy

>> BIOS (i.e., a CSM), and allocating the 64-bit BARs above 4 GB may put

>> them out of reach.

>>

>> Of course, none of this matters on ARM, and so we can unconditionally

>> override this decision. So take the OVMF implementation of the

>> IncompatiblePciDeviceSupportDxe driver, rip out the bits that care

>> about the presence of a legacy BIOS, and wire it up to the ArmPkg PCI

>> PCDs.

> 

> While I agree this solves a real problem, we're now at a point where

> we're adding multiple new implementations of a library


s/library/protocol/

> to give

> standards-compliant platforms the ability to circumvent legacy

> workarounds in core code.

> 

> Should this not be the other way around?


Yes, it should be.

... IMO, the specific degradation logic in the PCI Bus driver is
simplistic; for example, if it finds a PCI oprom for the device, it
could parse it to see if there's a legacy BIOS image (too) in that
oprom, before summarily degrading the resource.

But, implementing that in PciBusDxe now, or even eliminating the quirk
completely, would require all downstreams to make up for it in their
IncompatiblePciDeviceSupportDxe drivers. Getting shortcuts into core
code is easy (initally), getting them out is not so much.

So, I completely agree with you, especially that we're abusing this
protocol as a resource allocation hint -- as the patches themselves say,
there's nothing "incompatible" about the devices we handle. But,
changing the status quo at this stage looks really impractical.

> And if not, is that not a policy decision that suggests the workaround

> belongs as a core library anyway?


Based on the PI spec (1.4a, vol 5, "11.5 Incompatible PCI Device Support
Protocol"), the protocol should be provided by a platform driver.

Now, PciHostBridgeDriverDxe is also a platform driver. I think it's
different from this case because PciHostBridgeDriverDxe has so much
common / shared logic that it could be meaningfully split into a
PciHostBridgeLib class, and a common driver base (linking against the
lib class). For IncompatiblePciDeviceSupportDxe however, at least on the
level that we work with it, there's zero common logic to factor out.
(Okay, yes, you could centralize one or two PcdGet() calls, and perhaps
one of the PCDs could be moved to a central .DEC file.)

I don't like this situation either, but it's how ABI specs designed for
the proprietary world work in practice, I guess.

Thanks
Laszlo
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Ard Biesheuvel Sept. 12, 2016, 1:06 p.m. | #3
On 12 September 2016 at 13:29, Laszlo Ersek <lersek@redhat.com> wrote:
> On 09/12/16 12:23, Leif Lindholm wrote:

>> On Mon, Sep 12, 2016 at 11:01:17AM +0100, Ard Biesheuvel wrote:

>>> By default, the generic PciBusDxe driver uses the presence of a ROM BAR

>>> as a hint that all MMIO BARs should be allocated in the 32-bit region,

>>> even the ones that could be allocated above 4 GB. The reason is that

>>> such a ROM BAR may contain code that runs in the context of a legacy

>>> BIOS (i.e., a CSM), and allocating the 64-bit BARs above 4 GB may put

>>> them out of reach.

>>>

>>> Of course, none of this matters on ARM, and so we can unconditionally

>>> override this decision. So take the OVMF implementation of the

>>> IncompatiblePciDeviceSupportDxe driver, rip out the bits that care

>>> about the presence of a legacy BIOS, and wire it up to the ArmPkg PCI

>>> PCDs.

>>

>> While I agree this solves a real problem, we're now at a point where

>> we're adding multiple new implementations of a library

>

> s/library/protocol/

>

>> to give

>> standards-compliant platforms the ability to circumvent legacy

>> workarounds in core code.

>>

>> Should this not be the other way around?

>

> Yes, it should be.

>

> ... IMO, the specific degradation logic in the PCI Bus driver is

> simplistic; for example, if it finds a PCI oprom for the device, it

> could parse it to see if there's a legacy BIOS image (too) in that

> oprom, before summarily degrading the resource.

>

> But, implementing that in PciBusDxe now, or even eliminating the quirk

> completely, would require all downstreams to make up for it in their

> IncompatiblePciDeviceSupportDxe drivers. Getting shortcuts into core

> code is easy (initally), getting them out is not so much.

>


True, but I have sent out a patch nonetheless. Let's at least have the
discussion.

> So, I completely agree with you, especially that we're abusing this

> protocol as a resource allocation hint -- as the patches themselves say,

> there's nothing "incompatible" about the devices we handle. But,

> changing the status quo at this stage looks really impractical.

>

>> And if not, is that not a policy decision that suggests the workaround

>> belongs as a core library anyway?

>

> Based on the PI spec (1.4a, vol 5, "11.5 Incompatible PCI Device Support

> Protocol"), the protocol should be provided by a platform driver.

>

> Now, PciHostBridgeDriverDxe is also a platform driver. I think it's

> different from this case because PciHostBridgeDriverDxe has so much

> common / shared logic that it could be meaningfully split into a

> PciHostBridgeLib class, and a common driver base (linking against the

> lib class). For IncompatiblePciDeviceSupportDxe however, at least on the

> level that we work with it, there's zero common logic to factor out.

> (Okay, yes, you could centralize one or two PcdGet() calls, and perhaps

> one of the PCDs could be moved to a central .DEC file.)

>

> I don't like this situation either, but it's how ABI specs designed for

> the proprietary world work in practice, I guess.

>

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Patch

diff --git a/ArmPkg/Drivers/IncompatiblePciDeviceSupportDxe/IncompatiblePciDeviceSupport.c b/ArmPkg/Drivers/IncompatiblePciDeviceSupportDxe/IncompatiblePciDeviceSupport.c
new file mode 100644
index 000000000000..3f2869f8f3c2
--- /dev/null
+++ b/ArmPkg/Drivers/IncompatiblePciDeviceSupportDxe/IncompatiblePciDeviceSupport.c
@@ -0,0 +1,223 @@ 
+/** @file
+  A simple DXE_DRIVER that causes the PCI Bus UEFI_DRIVER to allocate 64-bit
+  MMIO BARs above 4 GB, regardless of option ROM availability, conserving
+  32-bit MMIO aperture for 32-bit BARs.
+
+  Copyright (C) 2016, Red Hat, Inc.
+  Copyright (C) 2016, Linaro, Ltd.
+
+  This program and the accompanying materials are licensed and made available
+  under the terms and conditions of the BSD License which accompanies this
+  distribution. The full text of the license may be found at
+  http://opensource.org/licenses/bsd-license.php
+
+  THE PROGRAM IS DISTRIBUTED UNDER THE BSD LICENSE ON AN "AS IS" BASIS, WITHOUT
+  WARRANTIES OR REPRESENTATIONS OF ANY KIND, EITHER EXPRESS OR IMPLIED.
+**/
+
+#include <IndustryStandard/Acpi10.h>
+#include <IndustryStandard/Pci22.h>
+
+#include <Library/DebugLib.h>
+#include <Library/MemoryAllocationLib.h>
+#include <Library/PcdLib.h>
+#include <Library/UefiBootServicesTableLib.h>
+
+#include <Protocol/IncompatiblePciDeviceSupport.h>
+
+//
+// The protocol interface this driver produces.
+//
+STATIC
+EFI_INCOMPATIBLE_PCI_DEVICE_SUPPORT_PROTOCOL    mIncompatiblePciDeviceSupport;
+
+//
+// Configuration template for the CheckDevice() protocol member function.
+//
+// Refer to Table 20 "ACPI 2.0 & 3.0 QWORD Address Space Descriptor Usage" in
+// the Platform Init 1.4a Spec, Volume 5.
+//
+// This structure is interpreted by the UpdatePciInfo() function in the edk2
+// PCI Bus UEFI_DRIVER.
+//
+#pragma pack (1)
+typedef struct {
+  EFI_ACPI_ADDRESS_SPACE_DESCRIPTOR AddressSpaceDesc;
+  EFI_ACPI_END_TAG_DESCRIPTOR       EndDesc;
+} MMIO64_PREFERENCE;
+#pragma pack ()
+
+STATIC CONST MMIO64_PREFERENCE mConfiguration = {
+  //
+  // AddressSpaceDesc
+  //
+  {
+    ACPI_ADDRESS_SPACE_DESCRIPTOR,                 // Desc
+    (UINT16)(                                      // Len
+      sizeof (EFI_ACPI_ADDRESS_SPACE_DESCRIPTOR) -
+      OFFSET_OF (
+        EFI_ACPI_ADDRESS_SPACE_DESCRIPTOR,
+        ResType
+        )
+      ),
+    ACPI_ADDRESS_SPACE_TYPE_MEM,                   // ResType
+    PCI_ACPI_UNUSED,                               // GenFlag
+    PCI_ACPI_UNUSED,                               // SpecificFlag
+    64,                                            // AddrSpaceGranularity:
+                                                   //   aperture selection hint
+                                                   //   for BAR allocation
+    PCI_ACPI_UNUSED,                               // AddrRangeMin
+    PCI_BAR_OLD_ALIGN,                             // AddrRangeMax:
+                                                   //   no special alignment
+                                                   //   for affected BARs
+    PCI_BAR_ALL,                                   // AddrTranslationOffset:
+                                                   //   hint covers all
+                                                   //   eligible BARs
+    PCI_BAR_NOCHANGE                               // AddrLen:
+                                                   //   use probed BAR size
+  },
+  //
+  // EndDesc
+  //
+  {
+    ACPI_END_TAG_DESCRIPTOR,                       // Desc
+    0                                              // Checksum: to be ignored
+  }
+};
+
+
+/**
+  Returns a list of ACPI resource descriptors that detail the special resource
+  configuration requirements for an incompatible PCI device.
+
+  Prior to bus enumeration, the PCI bus driver will look for the presence of
+  the EFI_INCOMPATIBLE_PCI_DEVICE_SUPPORT_PROTOCOL. Only one instance of this
+  protocol can be present in the system. For each PCI device that the PCI bus
+  driver discovers, the PCI bus driver calls this function with the device's
+  vendor ID, device ID, revision ID, subsystem vendor ID, and subsystem device
+  ID. If the VendorId, DeviceId, RevisionId, SubsystemVendorId, or
+  SubsystemDeviceId value is set to (UINTN)-1, that field will be ignored. The
+  ID values that are not (UINTN)-1 will be used to identify the current device.
+
+  This function will only return EFI_SUCCESS. However, if the device is an
+  incompatible PCI device, a list of ACPI resource descriptors will be returned
+  in Configuration. Otherwise, NULL will be returned in Configuration instead.
+  The PCI bus driver does not need to allocate memory for Configuration.
+  However, it is the PCI bus driver's responsibility to free it. The PCI bus
+  driver then can configure this device with the information that is derived
+  from this list of resource nodes, rather than the result of BAR probing.
+
+  Only the following two resource descriptor types from the ACPI Specification
+  may be used to describe the incompatible PCI device resource requirements:
+  - QWORD Address Space Descriptor (ACPI 2.0, section 6.4.3.5.1; also ACPI 3.0)
+  - End Tag (ACPI 2.0, section 6.4.2.8; also ACPI 3.0)
+
+  The QWORD Address Space Descriptor can describe memory, I/O, and bus number
+  ranges for dynamic or fixed resources. The configuration of a PCI root bridge
+  is described with one or more QWORD Address Space Descriptors, followed by an
+  End Tag. See the ACPI Specification for details on the field values.
+
+  @param[in]  This                Pointer to the
+                                  EFI_INCOMPATIBLE_PCI_DEVICE_SUPPORT_PROTOCOL
+                                  instance.
+
+  @param[in]  VendorId            A unique ID to identify the manufacturer of
+                                  the PCI device.  See the Conventional PCI
+                                  Specification 3.0 for details.
+
+  @param[in]  DeviceId            A unique ID to identify the particular PCI
+                                  device. See the Conventional PCI
+                                  Specification 3.0 for details.
+
+  @param[in]  RevisionId          A PCI device-specific revision identifier.
+                                  See the Conventional PCI Specification 3.0
+                                  for details.
+
+  @param[in]  SubsystemVendorId   Specifies the subsystem vendor ID. See the
+                                  Conventional PCI Specification 3.0 for
+                                  details.
+
+  @param[in]  SubsystemDeviceId   Specifies the subsystem device ID. See the
+                                  Conventional PCI Specification 3.0 for
+                                  details.
+
+  @param[out] Configuration       A list of ACPI resource descriptors that
+                                  detail the configuration requirement.
+
+  @retval EFI_SUCCESS   The function always returns EFI_SUCCESS.
+**/
+STATIC
+EFI_STATUS
+EFIAPI
+CheckDevice (
+  IN  EFI_INCOMPATIBLE_PCI_DEVICE_SUPPORT_PROTOCOL  *This,
+  IN  UINTN                                         VendorId,
+  IN  UINTN                                         DeviceId,
+  IN  UINTN                                         RevisionId,
+  IN  UINTN                                         SubsystemVendorId,
+  IN  UINTN                                         SubsystemDeviceId,
+  OUT VOID                                          **Configuration
+  )
+{
+  //
+  // Unlike the general description of this protocol member suggests, there is
+  // nothing incompatible about the PCI devices that we'll match here. We'll
+  // match all PCI devices, and generate exactly one QWORD Address Space
+  // Descriptor for each. That descriptor will instruct the PCI Bus UEFI_DRIVER
+  // not to degrade 64-bit MMIO BARs for the device, even if a PCI option ROM
+  // BAR is present on the device.
+  //
+
+  //
+  // This member function is mis-specified actually: it is supposed to allocate
+  // memory, but as specified, it could not return an error status. Thankfully,
+  // the edk2 PCI Bus UEFI_DRIVER actually handles error codes; see the
+  // UpdatePciInfo() function.
+  //
+  *Configuration = AllocateCopyPool (sizeof mConfiguration, &mConfiguration);
+  if (*Configuration == NULL) {
+    DEBUG ((EFI_D_WARN,
+      "%a: 64-bit MMIO BARs may be degraded for PCI 0x%04x:0x%04x (rev %d)\n",
+      __FUNCTION__, (UINT32)VendorId, (UINT32)DeviceId, (UINT8)RevisionId));
+    return EFI_OUT_OF_RESOURCES;
+  }
+  return EFI_SUCCESS;
+}
+
+
+/**
+  Entry point for this driver.
+
+  @param[in] ImageHandle  Image handle of this driver.
+  @param[in] SystemTable  Pointer to SystemTable.
+
+  @retval EFI_SUCESS       Driver has loaded successfully.
+  @retval EFI_UNSUPPORTED  PCI resource allocation has been disabled.
+  @retval EFI_UNSUPPORTED  There is no 64-bit PCI MMIO aperture.
+  @return                  Error codes from lower level functions.
+
+**/
+EFI_STATUS
+EFIAPI
+DriverInitialize (
+  IN EFI_HANDLE       ImageHandle,
+  IN EFI_SYSTEM_TABLE *SystemTable
+  )
+{
+  //
+  // If the PCI Bus driver is not supposed to allocate resources, then it makes
+  // no sense to install a protocol that influences the resource allocation.
+  //
+  // Similarly, if there is no 64-bit PCI MMIO aperture, then 64-bit MMIO BARs
+  // have to be allocated under 4 GB unconditionally.
+  //
+  if (PcdGetBool (PcdPciDisableBusEnumeration) ||
+      PcdGet64 (PcdPciMmio64Size) == 0) {
+    return EFI_UNSUPPORTED;
+  }
+
+  mIncompatiblePciDeviceSupport.CheckDevice = CheckDevice;
+  return gBS->InstallMultipleProtocolInterfaces (&ImageHandle,
+                &gEfiIncompatiblePciDeviceSupportProtocolGuid,
+                &mIncompatiblePciDeviceSupport, NULL);
+}
diff --git a/ArmPkg/Drivers/IncompatiblePciDeviceSupportDxe/IncompatiblePciDeviceSupport.inf b/ArmPkg/Drivers/IncompatiblePciDeviceSupportDxe/IncompatiblePciDeviceSupport.inf
new file mode 100644
index 000000000000..0bda20199d51
--- /dev/null
+++ b/ArmPkg/Drivers/IncompatiblePciDeviceSupportDxe/IncompatiblePciDeviceSupport.inf
@@ -0,0 +1,49 @@ 
+## @file
+# A simple DXE_DRIVER that causes the PCI Bus UEFI_DRIVER to allocate 64-bit
+# MMIO BARs above 4 GB, regardless of option ROM availability, conserving
+# 32-bit MMIO aperture for 32-bit BARs.
+#
+# Copyright (C) 2016, Red Hat, Inc.
+# Copyright (C) 2016, Linaro, Ltd.
+#
+# This program and the accompanying materials are licensed and made available
+# under the terms and conditions of the BSD License which accompanies this
+# distribution. The full text of the license may be found at
+# http://opensource.org/licenses/bsd-license.php
+#
+# THE PROGRAM IS DISTRIBUTED UNDER THE BSD LICENSE ON AN "AS IS" BASIS, WITHOUT
+# WARRANTIES OR REPRESENTATIONS OF ANY KIND, EITHER EXPRESS OR IMPLIED.
+##
+
+[Defines]
+  INF_VERSION                    = 0x00010005
+  BASE_NAME                      = IncompatiblePciDeviceSupportDxe
+  FILE_GUID                      = 18BCB238-844E-446B-AA2D-3DF38A25FA0F
+  MODULE_TYPE                    = DXE_DRIVER
+  VERSION_STRING                 = 1.0
+  ENTRY_POINT                    = DriverInitialize
+
+[Sources]
+  IncompatiblePciDeviceSupport.c
+
+[Packages]
+  ArmPkg/ArmPkg.dec
+  MdeModulePkg/MdeModulePkg.dec
+  MdePkg/MdePkg.dec
+
+[LibraryClasses]
+  DebugLib
+  MemoryAllocationLib
+  PcdLib
+  UefiBootServicesTableLib
+  UefiDriverEntryPoint
+
+[Protocols]
+  gEfiIncompatiblePciDeviceSupportProtocolGuid               ## PRODUCES
+
+[Pcd]
+  gArmTokenSpaceGuid.PcdPciMmio64Size                        ## CONSUMES
+  gEfiMdeModulePkgTokenSpaceGuid.PcdPciDisableBusEnumeration ## CONSUMES
+
+[Depex]
+  TRUE