KVM: x86: document AVIC/APICv inhibit reasons

These days there are too many AVIC/APICv inhibit
reasons, and it doesn't hurt to have some documentation
for them.

Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
Message-Id: <20220606180829.102503-2-mlevitsk@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 3a240a6..1f9e47b 100644 (file)
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -1047,14 +1047,69 @@ struct kvm_x86_msr_filter {
 };
 
 enum kvm_apicv_inhibit {
+
+       /********************************************************************/
+       /* INHIBITs that are relevant to both Intel's APICv and AMD's AVIC. */
+       /********************************************************************/
+
+       /*
+        * APIC acceleration is disabled by a module parameter
+        * and/or not supported in hardware.
+        */
        APICV_INHIBIT_REASON_DISABLE,
+
+       /*
+        * APIC acceleration is inhibited because AutoEOI feature is
+        * being used by a HyperV guest.
+        */
        APICV_INHIBIT_REASON_HYPERV,
+
+       /*
+        * APIC acceleration is inhibited because the userspace didn't yet
+        * enable the kernel/split irqchip.
+        */
+       APICV_INHIBIT_REASON_ABSENT,
+
+       /* APIC acceleration is inhibited because KVM_GUESTDBG_BLOCKIRQ
+        * (out of band, debug measure of blocking all interrupts on this vCPU)
+        * was enabled, to avoid AVIC/APICv bypassing it.
+        */
+       APICV_INHIBIT_REASON_BLOCKIRQ,
+
+       /******************************************************/
+       /* INHIBITs that are relevant only to the AMD's AVIC. */
+       /******************************************************/
+
+       /*
+        * AVIC is inhibited on a vCPU because it runs a nested guest.
+        *
+        * This is needed because unlike APICv, the peers of this vCPU
+        * cannot use the doorbell mechanism to signal interrupts via AVIC when
+        * a vCPU runs nested.
+        */
        APICV_INHIBIT_REASON_NESTED,
+
+       /*
+        * On SVM, the wait for the IRQ window is implemented with pending vIRQ,
+        * which cannot be injected when the AVIC is enabled, thus AVIC
+        * is inhibited while KVM waits for IRQ window.
+        */
        APICV_INHIBIT_REASON_IRQWIN,
+
+       /*
+        * PIT (i8254) 're-inject' mode, relies on EOI intercept,
+        * which AVIC doesn't support for edge triggered interrupts.
+        */
        APICV_INHIBIT_REASON_PIT_REINJ,
+
+       /*
+        * AVIC is inhibited because the guest has x2apic in its CPUID.
+        */
        APICV_INHIBIT_REASON_X2APIC,
-       APICV_INHIBIT_REASON_BLOCKIRQ,
-       APICV_INHIBIT_REASON_ABSENT,
+
+       /*
+        * AVIC is disabled because SEV doesn't support it.
+        */
        APICV_INHIBIT_REASON_SEV,
 };