[15/67] KVM: SVM: Drop vcpu_svm's pointless avic_backing_page field

Commit Message

Sean Christopherson April 4, 2025, 7:38 p.m. UTC

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.

Signed-off-by: Sean Christopherson <[e-mail]>
---
 arch/x86/kvm/svm/avic.c | 6 ++----
 arch/x86/kvm/svm/svm.h  | 1 -
 2 files changed, 2 insertions(+), 5 deletions(-)

Patch

diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
index a1f4a08d35f5..c8ba2ce4cfd8 100644
--- a/arch/x86/kvm/svm/avic.c
+++ b/arch/x86/kvm/svm/avic.c
@@ -245,7 +245,7 @@  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)
@@ -290,7 +290,7 @@  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)) {
@@ -307,8 +307,6 @@  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 294d5594c724..1cc4e145577c 100644
--- a/arch/x86/kvm/svm/svm.h
+++ b/arch/x86/kvm/svm/svm.h
@@ -301,7 +301,6 @@  struct vcpu_svm {
 
 	u32 ldr_reg;
 	u32 dfr_reg;
-	struct page *avic_backing_page;
 	u64 *avic_physical_id_cache;
 
 	/*