[PATCH 1/1] KVM: x86: Allow emulation of EOI writes with AVIC enabled

[Alejandro Jimenez]

According to section 15.29.9.2 - AVIC Access to un-accelerated vAPIC register
of the AMD APM [1]:

"A guest access to an APIC register that is not accelerated by AVIC results in
a #VMEXIT with the exit code of AVIC_NOACCEL. This fault is also generated if
an EOI is attempted when the highest priority in-service interrupt is set for
level-triggered mode."

This is also stated in Table 15-22 - Guest vAPIC Register Access Behavior,
confirming that AVIC hardware traps on EOI writes for level triggered
interrupts, and leading to the following call stack:

avic_unaccelerated_access_interception()
-> avic_unaccel_trap_write()
  -> kvm_apic_write_nodecode()
    -> kvm_lapic_msr_read()
      -> kvm_lapic_reg_read()

In kvm_lapic_reg_read(), the APIC_EOI offset (0xb0) is not allowed as valid, so
the error returned triggers the assertion introduced by 'commit 70c8327c11c6
("KVM: x86: Bug the VM if an accelerated x2APIC trap occurs on a "bad" reg")'
and kills the VM.

Add APIC_EOI offset to the valid mask in kvm_lapic_reg_read() to allow the
emulation of EOI behavior for level triggered interrupts.

[1] https://www.amd.com/system/files/TechDocs/24593.pdf

Fixes: 0105d1a52640 ("KVM: x2apic interface to lapic")
Signed-off-by: Alejandro Jimenez <alejandro.j.jimenez@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
---

I am unsure as to the proper commit to use for the Fixes: tag. Technically the
issue was introduced by the initial SVM AVIC commits in 2016, since they failed
to add the EOI offset to the valid mask.

To be safe, I used the commit that introduces the valid mask, but that is
somewhat misleading since at the time AVIC was not available, and I believe that
Intel posted interrupts implementation does not require access to EOI offset in
this code.

Please correct Fixes: tag if necessary.
---
 arch/x86/kvm/lapic.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index 9dda989a1cf0..61041fecfa89 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -1452,6 +1452,7 @@ static int kvm_lapic_reg_read(struct kvm_lapic *apic, u32 offset, int len,
 		APIC_REG_MASK(APIC_LVR) |
 		APIC_REG_MASK(APIC_TASKPRI) |
 		APIC_REG_MASK(APIC_PROCPRI) |
+		APIC_REG_MASK(APIC_EOI) |
 		APIC_REG_MASK(APIC_LDR) |
 		APIC_REG_MASK(APIC_DFR) |
 		APIC_REG_MASK(APIC_SPIV) |
-- 
2.34.2