| Message ID | 1488208162-26709-1-git-send-email-ard.biesheuvel@linaro.org |
|---|---|
| State | Accepted |
| Commit | 4b1dd5c457c8fb07937851c9dc0f38e48d130b14 |
| Headers | show |
On 02/27/17 16:09, Ard Biesheuvel wrote: > The only observeable effect of having PcdPerformanceLibraryPropertyMask > set to 1 is that a EfiReservedMemory region of 4 pages is allocated right > below the 4 GB mark. This region is out of bounds for the OS, which means > it is not even allowed to map it, to avoid speculative loads from it. > > On Linux, this may prevent the kernel from using a 1 GB block mapping for > this region, and instead it has to carve up the block as follows: > > 0xffffffff80000000-0xffffffffbe000000 992M PMD CON BLK > 0xffffffffbe000000-0xffffffffbfe00000 30M PMD BLK > 0xffffffffbfe00000-0xffffffffbfff0000 1984K PTE CON > 0xffffffffbfff0000-0xffffffffbfffc000 48K PTE > > where it would otherwise use a single 1 GB mapping (*), i.e., > > 0xffffffff80000000-0xffffffffc0000000 1G PGD > > To clarify, the latter is a single 8 byte entry in the top level page > table, whereas in the former case, we have two additional levels of > paging, requiring two extra 4 KB pages (on a 4 KB pagesize kernel). > > The real cost, however, is the TLB footprint, which goes up from a > single entry to a number between 90 and 1020, depending on whether > contiguous hints are honoured by the hardware. > > So let's remove PcdPerformanceLibraryPropertyMask until we find a reason > why we need it. > > (*) provided that no other allocations were deliberately located right > below the 4 GB mark, and that we are running with more than 3 GB of > memory, in which case most allocations will be over 4 GB, given EDK2's > default top-down allocation policy. > > Contributed-under: TianoCore Contribution Agreement 1.0 > Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org> > --- > v2: remove PCD altogether rather than explicitly set it to its default > value of 0 > > ArmVirtPkg/ArmVirt.dsc.inc | 1 - > 1 file changed, 1 deletion(-) > > diff --git a/ArmVirtPkg/ArmVirt.dsc.inc b/ArmVirtPkg/ArmVirt.dsc.inc > index 8e3d298723f1..cc09d38910a2 100644 > --- a/ArmVirtPkg/ArmVirt.dsc.inc > +++ b/ArmVirtPkg/ArmVirt.dsc.inc > @@ -295,7 +295,6 @@ [PcdsFixedAtBuild.common] > gEfiMdePkgTokenSpaceGuid.PcdMaximumLinkedListLength|1000000 > gEfiMdePkgTokenSpaceGuid.PcdSpinLockTimeout|10000000 > gEfiMdePkgTokenSpaceGuid.PcdDebugClearMemoryValue|0xAF > - gEfiMdePkgTokenSpaceGuid.PcdPerformanceLibraryPropertyMask|1 > gEfiMdePkgTokenSpaceGuid.PcdPostCodePropertyMask|0 > gEfiMdePkgTokenSpaceGuid.PcdUefiLibMaxPrintBufferSize|320 > > Reviewed-by: Laszlo Ersek <lersek@redhat.com> _______________________________________________ edk2-devel mailing list edk2-devel@lists.01.org https://lists.01.org/mailman/listinfo/edk2-devel
diff --git a/ArmVirtPkg/ArmVirt.dsc.inc b/ArmVirtPkg/ArmVirt.dsc.inc index 8e3d298723f1..cc09d38910a2 100644 --- a/ArmVirtPkg/ArmVirt.dsc.inc +++ b/ArmVirtPkg/ArmVirt.dsc.inc @@ -295,7 +295,6 @@ [PcdsFixedAtBuild.common] gEfiMdePkgTokenSpaceGuid.PcdMaximumLinkedListLength|1000000 gEfiMdePkgTokenSpaceGuid.PcdSpinLockTimeout|10000000 gEfiMdePkgTokenSpaceGuid.PcdDebugClearMemoryValue|0xAF - gEfiMdePkgTokenSpaceGuid.PcdPerformanceLibraryPropertyMask|1 gEfiMdePkgTokenSpaceGuid.PcdPostCodePropertyMask|0 gEfiMdePkgTokenSpaceGuid.PcdUefiLibMaxPrintBufferSize|320
The only observeable effect of having PcdPerformanceLibraryPropertyMask set to 1 is that a EfiReservedMemory region of 4 pages is allocated right below the 4 GB mark. This region is out of bounds for the OS, which means it is not even allowed to map it, to avoid speculative loads from it. On Linux, this may prevent the kernel from using a 1 GB block mapping for this region, and instead it has to carve up the block as follows: 0xffffffff80000000-0xffffffffbe000000 992M PMD CON BLK 0xffffffffbe000000-0xffffffffbfe00000 30M PMD BLK 0xffffffffbfe00000-0xffffffffbfff0000 1984K PTE CON 0xffffffffbfff0000-0xffffffffbfffc000 48K PTE where it would otherwise use a single 1 GB mapping (*), i.e., 0xffffffff80000000-0xffffffffc0000000 1G PGD To clarify, the latter is a single 8 byte entry in the top level page table, whereas in the former case, we have two additional levels of paging, requiring two extra 4 KB pages (on a 4 KB pagesize kernel). The real cost, however, is the TLB footprint, which goes up from a single entry to a number between 90 and 1020, depending on whether contiguous hints are honoured by the hardware. So let's remove PcdPerformanceLibraryPropertyMask until we find a reason why we need it. (*) provided that no other allocations were deliberately located right below the 4 GB mark, and that we are running with more than 3 GB of memory, in which case most allocations will be over 4 GB, given EDK2's default top-down allocation policy. Contributed-under: TianoCore Contribution Agreement 1.0 Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org> --- v2: remove PCD altogether rather than explicitly set it to its default value of 0 ArmVirtPkg/ArmVirt.dsc.inc | 1 - 1 file changed, 1 deletion(-) -- 2.7.4 _______________________________________________ edk2-devel mailing list edk2-devel@lists.01.org https://lists.01.org/mailman/listinfo/edk2-devel