Truncate the vcpu_id, a.k.a. x2APIC ID, to an 8-bit value when comparing
it against the xAPIC ID to avoid false positives (sort of) on systems
with >255 CPUs, i.e. with IDs that don't fit into a u8. The intent of
APIC_ID_MODIFIED is to inhibit APICv/AVIC when the xAPIC is changed from
it's original value,
The mismatch isn't technically a false positive, as architecturally the
xAPIC IDs do end up being aliased in this scenario, and neither APICv
nor AVIC correctly handles IPI virtualization when there is aliasing.
However, KVM already deliberately does not honor the aliasing behavior
that results when an x2APIC ID gets truncated to an xAPIC ID. I.e. the
resulting APICv/AVIC behavior is aligned with KVM's existing behavior
when KVM's x2APIC hotplug hack is effectively enabled.
If/when KVM provides a way to disable the hotplug hack, APICv/AVIC can
piggyback whatever logic disables the optimized APIC map (which is what
provides the hotplug hack), i.e. so that KVM's optimized map and APIC
virtualization yield the same behavior.
For now, fix the immediate problem of APIC virtualization being disabled
for large VMs, which is a much more pressing issue than ensuring KVM
honors architectural behavior for APIC ID aliasing.
Fixes: 3743c2f02517 ("KVM: x86: inhibit APICv/AVIC on changes to APIC ID or APIC base")
Reported-by: Suravee Suthikulpanit <[email protected]>
Cc: Maxim Levitsky <[email protected]>
Cc: [email protected]
Signed-off-by: Sean Christopherson <[email protected]>
---
arch/x86/kvm/lapic.c | 7 ++++++-
1 file changed, 6 insertions(+), 1 deletion(-)
diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index 251856ba0750..2503f162eb95 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -2078,7 +2078,12 @@ static void kvm_lapic_xapic_id_updated(struct kvm_lapic *apic)
if (KVM_BUG_ON(apic_x2apic_mode(apic), kvm))
return;
- if (kvm_xapic_id(apic) == apic->vcpu->vcpu_id)
+ /*
+ * Deliberately truncate the vCPU ID when detecting a modified APIC ID
+ * to avoid false positives if the vCPU ID, i.e. x2APIC ID, is a 32-bit
+ * value.
+ */
+ if (kvm_xapic_id(apic) == (u8)apic->vcpu->vcpu_id)
return;
kvm_set_apicv_inhibit(apic->vcpu->kvm, APICV_INHIBIT_REASON_APIC_ID_MODIFIED);
--
2.38.0.rc1.362.ged0d419d3c-goog
On Sat, 2022-10-01 at 00:58 +0000, Sean Christopherson wrote:
> Truncate the vcpu_id, a.k.a. x2APIC ID, to an 8-bit value when comparing
> it against the xAPIC ID to avoid false positives (sort of) on systems
> with >255 CPUs, i.e. with IDs that don't fit into a u8. The intent of
> APIC_ID_MODIFIED is to inhibit APICv/AVIC when the xAPIC is changed from
> it's original value,
>
> The mismatch isn't technically a false positive, as architecturally the
> xAPIC IDs do end up being aliased in this scenario, and neither APICv
> nor AVIC correctly handles IPI virtualization when there is aliasing.
> However, KVM already deliberately does not honor the aliasing behavior
> that results when an x2APIC ID gets truncated to an xAPIC ID. I.e. the
> resulting APICv/AVIC behavior is aligned with KVM's existing behavior
> when KVM's x2APIC hotplug hack is effectively enabled.
>
> If/when KVM provides a way to disable the hotplug hack, APICv/AVIC can
> piggyback whatever logic disables the optimized APIC map (which is what
> provides the hotplug hack), i.e. so that KVM's optimized map and APIC
> virtualization yield the same behavior.
>
> For now, fix the immediate problem of APIC virtualization being disabled
> for large VMs, which is a much more pressing issue than ensuring KVM
> honors architectural behavior for APIC ID aliasing.
>
> Fixes: 3743c2f02517 ("KVM: x86: inhibit APICv/AVIC on changes to APIC ID or APIC base")
> Reported-by: Suravee Suthikulpanit <[email protected]>
> Cc: Maxim Levitsky <[email protected]>
> Cc: [email protected]
> Signed-off-by: Sean Christopherson <[email protected]>
> ---
> arch/x86/kvm/lapic.c | 7 ++++++-
> 1 file changed, 6 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index 251856ba0750..2503f162eb95 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -2078,7 +2078,12 @@ static void kvm_lapic_xapic_id_updated(struct kvm_lapic *apic)
> if (KVM_BUG_ON(apic_x2apic_mode(apic), kvm))
> return;
>
> - if (kvm_xapic_id(apic) == apic->vcpu->vcpu_id)
> + /*
> + * Deliberately truncate the vCPU ID when detecting a modified APIC ID
> + * to avoid false positives if the vCPU ID, i.e. x2APIC ID, is a 32-bit
> + * value.
> + */
> + if (kvm_xapic_id(apic) == (u8)apic->vcpu->vcpu_id)
> return;
>
> kvm_set_apicv_inhibit(apic->vcpu->kvm, APICV_INHIBIT_REASON_APIC_ID_MODIFIED);
Reviewed-by: Maxim Levitsky <[email protected]>
Best regards,
Maxim Levitsky