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KVM: SVM: Drop vcpu_svm's pointless avic_backing_page field

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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>
Reviewed-by: Naveen N Rao (AMD) <naveen@kernel.org>
Link: https://lore.kernel.org/r/20250611224604.313496-13-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>
This commit is contained in:
Sean Christopherson
2025-06-11 15:45:14 -07:00
parent 3338c639da
commit d8527f133c
2 changed files with 2 additions and 5 deletions
Download Patch File Download Diff File Expand all files Collapse all files

6
arch/x86/kvm/svm/avic.c
View File

@@ -237,7 +237,7 @@ 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)
@@ -282,7 +282,7 @@ 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)) {
@@ -299,8 +299,6 @@ 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)

1
arch/x86/kvm/svm/svm.h
View File

@@ -306,7 +306,6 @@ struct vcpu_svm {
u32 ldr_reg;
u32 dfr_reg;
struct page *avic_backing_page;
u64 *avic_physical_id_cache;
/*