[PATCH v6 13/17] KVM: SVM: Introduce hybrid-AVIC mode

Suravee Suthikulpanit
2022-05-19T10:29:10Z

Currently, AVIC is inhibited when booting a VM w/ x2APIC support.
because AVIC cannot virtualize x2APIC MSR register accesses.
However, the AVIC doorbell can be used to accelerate interrupt
injection into a running vCPU, while all guest accesses to x2APIC MSRs
will be intercepted and emulated by KVM.

With hybrid-AVIC support, the APICV_INHIBIT_REASON_X2APIC is
no longer enforced.

Suggested-by: Maxim Levitsky <mlevitsk@redhat.com>
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
---
arch/x86/include/asm/kvm_host.h | 1 -
arch/x86/kvm/svm/avic.c | 13 +++++++++++--
arch/x86/kvm/svm/svm.c | 9 ---------
3 files changed, 11 insertions(+), 12 deletions(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index c59fea4bdb6e..da03111b05f6 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -1051,7 +1051,6 @@ enum kvm_apicv_inhibit {
APICV_INHIBIT_REASON_NESTED,
APICV_INHIBIT_REASON_IRQWIN,
APICV_INHIBIT_REASON_PIT_REINJ,
- APICV_INHIBIT_REASON_X2APIC,
APICV_INHIBIT_REASON_BLOCKIRQ,
APICV_INHIBIT_REASON_ABSENT,
APICV_INHIBIT_REASON_SEV,
diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
index 2d9455338b1f..bac876bb1cf1 100644
--- a/arch/x86/kvm/svm/avic.c
+++ b/arch/x86/kvm/svm/avic.c
@@ -71,12 +71,22 @@ static void avic_activate_vmcb(struct vcpu_svm *svm)
vmcb->control.avic_physical_id &= ~AVIC_PHYSICAL_MAX_INDEX_MASK;

vmcb->control.int_ctl |= AVIC_ENABLE_MASK;
- if (apic_x2apic_mode(svm->vcpu.arch.apic)) {
+
+ /* Note:
+ * KVM can support hybrid-AVIC mode, where KVM emulates x2APIC
+ * MSR accesses, while interrupt injection to a running vCPU
+ * can be achieved using AVIC doorbell. The AVIC hardware still
+ * accelerate MMIO accesses, but this does not cause any harm
+ * as the guest is not supposed to access xAPIC mmio when uses x2APIC.
+ */
+ if (apic_x2apic_mode(svm->vcpu.arch.apic) &&
+ (avic_mode == AVIC_MODE_X2)) {
vmcb->control.int_ctl |= X2APIC_MODE_MASK;
vmcb->control.avic_physical_id |= X2AVIC_MAX_PHYSICAL_ID;
/* Disabling MSR intercept for x2APIC registers */
svm_set_x2apic_msr_interception(svm, false);
} else {
+ /* For xAVIC and hybrid-xAVIC modes */
vmcb->control.avic_physical_id |= AVIC_MAX_PHYSICAL_ID;
/* Enabling MSR intercept for x2APIC registers */
svm_set_x2apic_msr_interception(svm, true);
@@ -978,7 +988,6 @@ bool avic_check_apicv_inhibit_reasons(enum kvm_apicv_inhibit reason)
BIT(APICV_INHIBIT_REASON_NESTED) |
BIT(APICV_INHIBIT_REASON_IRQWIN) |
BIT(APICV_INHIBIT_REASON_PIT_REINJ) |
- BIT(APICV_INHIBIT_REASON_X2APIC) |
BIT(APICV_INHIBIT_REASON_BLOCKIRQ) |
BIT(APICV_INHIBIT_REASON_SEV);

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 0ec2444c342d..e04a133b98d0 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -4061,7 +4061,6 @@ static void svm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
{
struct vcpu_svm *svm = to_svm(vcpu);
struct kvm_cpuid_entry2 *best;
- struct kvm *kvm = vcpu->kvm;

vcpu->arch.xsaves_enabled = guest_cpuid_has(vcpu, X86_FEATURE_XSAVE) &&
boot_cpu_has(X86_FEATURE_XSAVE) &&
@@ -4093,14 +4092,6 @@ static void svm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
vcpu->arch.reserved_gpa_bits &= ~(1UL << (best->ebx & 0x3f));
}

- if (kvm_vcpu_apicv_active(vcpu)) {
- /*
- * AVIC does not work with an x2APIC mode guest. If the X2APIC feature
- * is exposed to the guest, disable AVIC.
- */
- if (guest_cpuid_has(vcpu, X86_FEATURE_X2APIC))
- kvm_set_apicv_inhibit(kvm, APICV_INHIBIT_REASON_X2APIC);
- }
init_vmcb_after_set_cpuid(vcpu);
}

--
2.25.1