[PATCH v3 11/62] KVM: SVM: Drop vcpu_svm's pointless avic_backing_page field

Sean Christopherson
2025-06-11T22:47:16Z

Drop vcpu_svm's avic_backing_page pointer and instead grab the physical
address of KVM's vAPIC page directly from the source. Getting a physical
address from a kernel virtual address is not an expensive operation, and
getting the physical address from a struct page is *more* expensive for
CONFIG_SPARSEMEM=y kernels. Regardless, none of the paths that consume
the address are hot paths, i.e. shaving cycles is not a priority.

Eliminating the "cache" means KVM doesn't have to worry about the cache
being invalid, which will simplify a future fix when dealing with vCPU IDs
that are too big.

WARN if KVM attempts to allocate a vCPU's AVIC backing page without an
in-kernel local APIC. avic_init_vcpu() bails early if the APIC is not
in-kernel, and KVM disallows enabling an in-kernel APIC after vCPUs have
been created, i.e. it should be impossible to reach
avic_init_backing_page() without the vAPIC being allocated.

Tested-by: Sairaj Kodilkar <sarunkod@amd.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
arch/x86/kvm/svm/avic.c | 6 ++----
arch/x86/kvm/svm/svm.h | 1 -
2 files changed, 2 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
index c36f7db9252e..ab228872a19b 100644
--- a/arch/x86/kvm/svm/avic.c
+++ b/arch/x86/kvm/svm/avic.c
@@ -236,7 +236,7 @@ int avic_vm_init(struct kvm *kvm)

static phys_addr_t avic_get_backing_page_address(struct vcpu_svm *svm)
{
- return __sme_set(page_to_phys(svm->avic_backing_page));
+ return __sme_set(__pa(svm->vcpu.arch.apic->regs));
}

void avic_init_vmcb(struct vcpu_svm *svm, struct vmcb *vmcb)
@@ -281,7 +281,7 @@ static int avic_init_backing_page(struct kvm_vcpu *vcpu)
(id > X2AVIC_MAX_PHYSICAL_ID))
return -EINVAL;

- if (!vcpu->arch.apic->regs)
+ if (WARN_ON_ONCE(!vcpu->arch.apic->regs))
return -EINVAL;

if (kvm_apicv_activated(vcpu->kvm)) {
@@ -298,8 +298,6 @@ static int avic_init_backing_page(struct kvm_vcpu *vcpu)
return ret;
}

- svm->avic_backing_page = virt_to_page(vcpu->arch.apic->regs);
-
/* Setting AVIC backing page address in the phy APIC ID table */
entry = avic_get_physical_id_entry(vcpu, id);
if (!entry)
diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
index cc27877d69ae..1585288200f4 100644
--- a/arch/x86/kvm/svm/svm.h
+++ b/arch/x86/kvm/svm/svm.h
@@ -306,7 +306,6 @@ struct vcpu_svm {

u32 ldr_reg;
u32 dfr_reg;
- struct page *avic_backing_page;
u64 *avic_physical_id_cache;

/*
--
2.50.0.rc1.591.g9c95f17f64-goog