[PATCH] x86/apic: KVM: Use cpu_physical_id() to get APIC ID of running vCPU for AVIC

[PATCH] x86/apic: KVM: Use cpu_physical_id() to get APIC ID of running vCPU for AVIC

[Sean Christopherson]

Use cpu_physical_id() instead of default_cpu_present_to_apicid() when
getting the APIC ID of the pCPU on which a vCPU is running/loaded, as the
kernel has gone way off the rails if a vCPU is loaded on a pCPU that has
been physically removed from the system.  Even if the impossible were to
happen, the absolutely worst case scenario is that hardware will ring the
AIVC doorbell on the wrong pCPU, i.e. a severely broken system will
experience mild performance issues.

Kill off KVM's superfluous kvm_cpu_get_apicid() wrapper along with the
for-KVM export of default_cpu_present_to_apicid(), as they existed purely
for the wonky AVIC usage.

Cc: Kai Huang <kai.huang@intel.com>
Cc: Yosry Ahmed <yosry@kernel.org>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---

Tip tree folks, I'd like to take this through the kvm-x86 tree (in 7.3) for
obvious reasons.  I assume the odds of a conflict on the removal of
EXPORT_SYMBOL_FOR_KVM() are tiny.

 arch/x86/include/asm/kvm_host.h    | 10 ----------
 arch/x86/kernel/apic/apic_common.c |  1 -
 arch/x86/kvm/svm/avic.c            |  6 +++---
 3 files changed, 3 insertions(+), 14 deletions(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 3886b536c8a5..2389e43e2f82 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -2526,16 +2526,6 @@ static inline void kvm_arch_vcpu_unblocking(struct kvm_vcpu *vcpu)
 	kvm_x86_call(vcpu_unblocking)(vcpu);
 }
 
-static inline int kvm_cpu_get_apicid(int mps_cpu)
-{
-#ifdef CONFIG_X86_LOCAL_APIC
-	return default_cpu_present_to_apicid(mps_cpu);
-#else
-	WARN_ON_ONCE(1);
-	return BAD_APICID;
-#endif
-}
-
 int memslot_rmap_alloc(struct kvm_memory_slot *slot, unsigned long npages);
 
 #define KVM_CLOCK_VALID_FLAGS						\
diff --git a/arch/x86/kernel/apic/apic_common.c b/arch/x86/kernel/apic/apic_common.c
index 2ed3b5c88c7f..45e6b816353e 100644
--- a/arch/x86/kernel/apic/apic_common.c
+++ b/arch/x86/kernel/apic/apic_common.c
@@ -26,7 +26,6 @@ u32 default_cpu_present_to_apicid(int mps_cpu)
 	else
 		return BAD_APICID;
 }
-EXPORT_SYMBOL_FOR_KVM(default_cpu_present_to_apicid);
 
 /*
  * Set up the logical destination ID when the APIC operates in logical
diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
index 0726f88e679a..58e493a80cb0 100644
--- a/arch/x86/kvm/svm/avic.c
+++ b/arch/x86/kvm/svm/avic.c
@@ -460,8 +460,8 @@ void avic_ring_doorbell(struct kvm_vcpu *vcpu)
 	int cpu = READ_ONCE(vcpu->cpu);
 
 	if (cpu != get_cpu()) {
-		wrmsrq(MSR_AMD64_SVM_AVIC_DOORBELL, kvm_cpu_get_apicid(cpu));
-		trace_kvm_avic_doorbell(vcpu->vcpu_id, kvm_cpu_get_apicid(cpu));
+		wrmsrq(MSR_AMD64_SVM_AVIC_DOORBELL, cpu_physical_id(cpu));
+		trace_kvm_avic_doorbell(vcpu->vcpu_id, cpu_physical_id(cpu));
 	}
 	put_cpu();
 }
@@ -1013,7 +1013,7 @@ static void __avic_vcpu_load(struct kvm_vcpu *vcpu, int cpu,
 			     enum avic_vcpu_action action)
 {
 	struct kvm_svm *kvm_svm = to_kvm_svm(vcpu->kvm);
-	int h_physical_id = kvm_cpu_get_apicid(cpu);
+	int h_physical_id = cpu_physical_id(cpu);
 	struct vcpu_svm *svm = to_svm(vcpu);
 	unsigned long flags;
 	u64 entry;

base-commit: c1f7303302927f9cbf4efedf70f0512cde168c65
-- 
2.54.0.1136.gdb2ca164c4-goog

Re: [PATCH] x86/apic: KVM: Use cpu_physical_id() to get APIC ID of running vCPU for AVIC

[Yosry Ahmed]

On Fri, Jun 12, 2026 at 11:55 AM Sean Christopherson <seanjc@google.com> wrote:
>
> Use cpu_physical_id() instead of default_cpu_present_to_apicid() when
> getting the APIC ID of the pCPU on which a vCPU is running/loaded, as the
> kernel has gone way off the rails if a vCPU is loaded on a pCPU that has
> been physically removed from the system.  Even if the impossible were to
> happen, the absolutely worst case scenario is that hardware will ring the
> AIVC doorbell on the wrong pCPU, i.e. a severely broken system will
> experience mild performance issues.
>
> Kill off KVM's superfluous kvm_cpu_get_apicid() wrapper along with the
> for-KVM export of default_cpu_present_to_apicid(), as they existed purely
> for the wonky AVIC usage.
>
> Cc: Kai Huang <kai.huang@intel.com>
> Cc: Yosry Ahmed <yosry@kernel.org>
> Signed-off-by: Sean Christopherson <seanjc@google.com>

Took me a bit to realize cpu_physical_id() is the APIC ID, but yeah
this looks to be equivalent as long as the CPU is present, which as
you described should be true as long as we didn't screw up.
cpu_physical_id() seems to also be used by VMX anyway.

Reviewed-by: Yosry Ahmed <yosry@kernel.org>

Re: [PATCH] x86/apic: KVM: Use cpu_physical_id() to get APIC ID of running vCPU for AVIC

[Huang, Kai]

On Fri, 2026-06-12 at 11:54 -0700, Sean Christopherson wrote:
> Use cpu_physical_id() instead of default_cpu_present_to_apicid() when
> getting the APIC ID of the pCPU on which a vCPU is running/loaded, as the
> kernel has gone way off the rails if a vCPU is loaded on a pCPU that has
> been physically removed from the system.  Even if the impossible were to
> happen, the absolutely worst case scenario is that hardware will ring the
> AIVC doorbell on the wrong pCPU, i.e. a severely broken system will
> experience mild performance issues.
> 
> Kill off KVM's superfluous kvm_cpu_get_apicid() wrapper along with the
> for-KVM export of default_cpu_present_to_apicid(), as they existed purely
> for the wonky AVIC usage.
> 
> Cc: Kai Huang <kai.huang@intel.com>
> Cc: Yosry Ahmed <yosry@kernel.org>
> Signed-off-by: Sean Christopherson <seanjc@google.com>

Reviewed-by: Kai Huang <kai.huang@intel.com>

Re: [PATCH] x86/apic: KVM: Use cpu_physical_id() to get APIC ID of running vCPU for AVIC

[Naveen N Rao]

On Fri, Jun 12, 2026 at 11:54:59AM -0700, Sean Christopherson wrote:
> Use cpu_physical_id() instead of default_cpu_present_to_apicid() when
> getting the APIC ID of the pCPU on which a vCPU is running/loaded, as the
> kernel has gone way off the rails if a vCPU is loaded on a pCPU that has
> been physically removed from the system.

Or, has been unloaded from a pCPU that is then soft-offlined and 
physically hot unplugged before that vCPU ever got a chance to run 
again!

> Even if the impossible were to
> happen, the absolutely worst case scenario is that hardware will ring the
> AIVC doorbell on the wrong pCPU, i.e. a severely broken system will
> experience mild performance issues.

Indeed, sending a doorbell to BAD_APICID doesn't seem to do anything 
bad, so I think we are ok with this.

> 
> Kill off KVM's superfluous kvm_cpu_get_apicid() wrapper along with the
> for-KVM export of default_cpu_present_to_apicid(), as they existed purely
> for the wonky AVIC usage.
> 
> Cc: Kai Huang <kai.huang@intel.com>
> Cc: Yosry Ahmed <yosry@kernel.org>
> Signed-off-by: Sean Christopherson <seanjc@google.com>

Acked-by: Naveen N Rao (AMD) <naveen@kernel.org>


- Naveen

Re: [PATCH] x86/apic: KVM: Use cpu_physical_id() to get APIC ID of running vCPU for AVIC

[Paolo Bonzini]

Queued, thanks.

Paolo