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+KVM/ARM VGIC Forwarded Physical Interrupts
+==========================================
+
+The KVM/ARM code implements software support for the ARM Generic
+Interrupt Controller's (GIC's) hardware support for virtualization by
+allowing software to inject virtual interrupts to a VM, which the guest
+OS sees as regular interrupts.  The code is famously known as the VGIC.
+
+Some of these virtual interrupts, however, correspond to physical
+interrupts from real physical devices.  One example could be the
+architected timer, which itself supports virtualization, and therefore
+lets a guest OS program the hardware device directly to raise an
+interrupt at some point in time.  When such an interrupt is raised, the
+host OS initially handles the interrupt and must somehow signal this
+event as a virtual interrupt to the guest.  Another example could be a
+passthrough device, where the physical interrupts are initially handled
+by the host, but the device driver for the device lives in the guest OS
+and KVM must therefore somehow inject a virtual interrupt on behalf of
+the physical one to the guest OS.
+
+These virtual interrupts corresponding to a physical interrupt on the
+host are called forwarded physical interrupts, but are also sometimes
+referred to as 'virtualized physical interrupts' and 'mapped interrupts'.
+
+Forwarded physical interrupts are handled slightly differently compared
+to virtual interrupts generated purely by a software emulated device.
+
+
+The HW bit
+----------
+Virtual interrupts are signalled to the guest by programming the List
+Registers (LRs) on the GIC before running a VCPU.  The LR is programmed
+with the virtual IRQ number and the state of the interrupt (Pending,
+Active, or Pending+Active).  When the guest ACKs and EOIs a virtual
+interrupt, the LR state moves from Pending to Active, and finally to
+inactive.
+
+The LRs include an extra bit, called the HW bit.  When this bit is set,
+KVM must also program an additional field in the LR, the physical IRQ
+number, to link the virtual with the physical IRQ.
+
+When the HW bit is set, KVM must EITHER set the Pending OR the Active
+bit, never both at the same time.
+
+Setting the HW bit causes the hardware to deactivate the physical
+interrupt on the physical distributor when the guest deactivates the
+corresponding virtual interrupt.
+
+
+Forwarded Physical Interrupts Life Cycle
+----------------------------------------
+
+The state of forwarded physical interrupts is managed in the following way:
+
+  - The physical interrupt is acked by the host, and becomes active on
+    the physical distributor (*).
+  - KVM sets the LR.Pending bit, because this is the only way the GICV
+    interface is going to present it to the guest.
+  - LR.Pending will stay set as long as the guest has not acked the interrupt.
+  - LR.Pending transitions to LR.Active on the guest read of the IAR, as
+    expected.
+  - On guest EOI, the *physical distributor* active bit gets cleared,
+    but the LR.Active is left untouched (set).
+  - KVM clears the LR when on VM exits when the physical distributor
+    active state has been cleared.
+
+(*): The host handling is slightly more complicated.  For some devices
+(shared), KVM directly sets the active state on the physical distributor
+before entering the guest, and for some devices (non-shared) the host
+configures the GIC such that it does not deactivate the interrupt on
+host EOIs, but only performs a priority drop allowing the GIC to receive
+other interrupts and leaves the interrupt in the active state on the
+physical distributor.
+
+
+Forwarded Edge and Level Triggered PPIs and SPIs
+------------------------------------------------
+Forwarded physical interrupts injected should always be active on the
+physical distributor when injected to a guest.
+
+Level-triggered interrupts will keep the interrupt line to the GIC
+asserted, typically until the guest programs the device to deassert the
+line.  This means that the interrupt will remain pending on the physical
+distributor until the guest has reprogrammed the device.  Since we
+always run the VM with interrupts enabled on the CPU, a pending
+interrupt will exit the guest as soon as we switch into the guest,
+preventing the guest from ever making progress as the process repeats
+over and over.  Therefore, the active state on the physical distributor
+must be set when entering the guest, preventing the GIC from forwarding
+the pending interrupt to the CPU.  As soon as the guest deactivates
+(EOIs) the interrupt, the physical line is sampled by the hardware again
+and the host takes a new interrupt if and only if the physical line is
+still asserted.
+
+Edge-triggered interrupts do not exhibit the same problem with
+preventing guest execution that level-triggered interrupts do.  One
+option is to not use HW bit at all, and inject edge-triggered interrupts
+from a physical device as pure virtual interrupts.  But that would
+potentially slow down handling of the interrupt in the guest, because a
+physical interrupt occurring in the middle of the guest ISR would
+preempt the guest for the host to handle the interrupt.  Additionally,
+if you configure the system to handle interrupts on a separate physical
+core from that running your VCPU, you still have to interrupt the VCPU
+to queue the pending state onto the LR, even though the guest won't use
+this information until the guest ISR completes.  Therefore, the HW
+bit should always be set for forwarded edge-triggered interrupts.  With
+the HW bit set, the virtual interrupt is injected and additional
+physical interrupts occurring before the guest deactivates the interrupt
+simply mark the state on the physical distributor as Pending+Active.  As
+soon as the guest deactivates the interrupt, the host takes another
+interrupt if and only if there was a physical interrupt between
+injecting the forwarded interrupt to the guest the guest deactivating
+the interrupt.
+
+Consequently, whenever we schedule a VCPU with one or more LRs with the
+HW bit set, the interrupt must also be active on the physical
+distributor.
+
+
+Forwarded LPIs
+--------------
+LPIs, introduced in GICv3, are always edge-triggered and do not have an
+active state.  They become pending when a device signal them, and as
+soon as they are acked by the CPU, they are inactive again.
+
+It therefore doesn't make sense, and is not supported, to set the HW bit
+for physical LPIs that are forwarded to a VM as virtual interrupts,
+typically virtual SPIs.
+
+For LPIs, there is no other choice than to preempt the VCPU thread if
+necessary, and queue the pending state onto the LR.
+
+
+Putting It Together: The Architected Timer
+------------------------------------------
+The architected timer is a device that signals interrupts with level
+triggered semantics.  The timer hardware is directly accessed by VCPUs
+which program the timer to fire at some point in time.  Each VCPU on a
+system programs the timer to fire at different times, and therefore the
+hardware is multiplexed between multiple VCPUs.  This is implemented by
+context-switching the timer state along with each VCPU thread.
+
+However, this means that a scenario like the following is entirely
+possible, and in fact, typical:
+
+1.  KVM runs the VCPU
+2.  The guest programs the time to fire in T+100
+3.  The guest is idle and calls WFI (wait-for-interrupts)
+4.  The hardware traps to the host
+5.  KVM stores the timer state to memory and disables the hardware timer
+6.  KVM schedules a soft timer to fire in T+(100 - time since step 2)
+7.  KVM puts the VCPU thread to sleep (on a waitqueue)
+8.  The soft timer fires, waking up the VCPU thread
+9.  KVM reprograms the timer hardware with the VCPU's values
+10. KVM marks the timer interrupt as active on the physical distributor
+11. KVM injects a forwarded physical interrupt to the guest
+12. KVM runs the VCPU
+
+Notice that KVM injects a forwarded physical interrupt in step 11 without
+the corresponding interrupt having actually fired on the host.  That is
+exactly why we mark the timer interrupt as active in step 10, because
+the active state on the physical distributor is part of the state
+belonging to the timer hardware, which is context-switched along with
+the VCPU thread.
+
+If the guest does not idle because it is busy, flow looks like this
+instead:
+
+1.  KVM runs the VCPU
+2.  The guest programs the time to fire in T+100
+4.  At T+100 the timer fires and a physical IRQ causes the VM to exit
+5.  With interrupts disabled on the CPU, KVM looks at the timer state
+    and injects a forwarded physical interrupt because it concludes the
+    timer has expired.
+6.  KVM marks the timer interrupt as active on the physical distributor
+7.  KVM runs the VCPU
+
+Notice that again the forwarded physical interrupt is injected to the
+guest without having actually been handled on the host.  In this case it
+is because the physical interrupt is forwarded to the guest before KVM
+enables physical interrupts on the CPU after exiting the guest.
