[PATCH 09/11] KVM: nSVM: Use kvm_rax_read() now that it's mode-aware

Sean Christopherson
2026-04-09T23:56:45Z

Now that kvm_rax_read() truncates the output value to 32 bits if the
vCPU isn't in 64-bit mode, use it instead of the more verbose (and very
technically slower) kvm_register_read().

Note! VMLOAD, VMSAVE, and VMRUN emulation are still technically buggy,
as they can use EAX (versus RAX) in 64-bit mode via an operand size
prefix. Don't bother trying to handle that case, as it would require
decoding the code stream, which would open an entirely different can of
worms, and in practice no sane guest would shove garbage into RAX[63:32]
and then execute VMLOAD/VMSAVE/VMRUN with just EAX.

No functional change intended.

Cc: Yosry Ahmed <yosry@kernel.org>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
arch/x86/kvm/svm/nested.c | 2 +-
arch/x86/kvm/svm/svm.c | 4 ++--
2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
index 00de9375c836..7bea5ad02805 100644
--- a/arch/x86/kvm/svm/nested.c
+++ b/arch/x86/kvm/svm/nested.c
@@ -1113,7 +1113,7 @@ int nested_svm_vmrun(struct kvm_vcpu *vcpu)
if (WARN_ON_ONCE(!svm->nested.initialized))
return -EINVAL;

- vmcb12_gpa = kvm_register_read(vcpu, VCPU_REGS_RAX);
+ vmcb12_gpa = kvm_rax_read(vcpu);
if (!page_address_valid(vcpu, vmcb12_gpa)) {
kvm_inject_gp(vcpu, 0);
return 1;
diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 0e2e7a803d64..79d5982cf294 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -2187,7 +2187,7 @@ static int intr_interception(struct kvm_vcpu *vcpu)

static int vmload_vmsave_interception(struct kvm_vcpu *vcpu, bool vmload)
{
- u64 vmcb12_gpa = kvm_register_read(vcpu, VCPU_REGS_RAX);
+ u64 vmcb12_gpa = kvm_rax_read(vcpu);
struct vcpu_svm *svm = to_svm(vcpu);
struct vmcb *vmcb12;
struct kvm_host_map map;
@@ -2295,7 +2295,7 @@ static int gp_interception(struct kvm_vcpu *vcpu)
if (nested_svm_check_permissions(vcpu))
return 1;

- if (!page_address_valid(vcpu, kvm_register_read(vcpu, VCPU_REGS_RAX)))
+ if (!page_address_valid(vcpu, kvm_rax_read(vcpu)))
goto reinject;

/*
--
2.53.0.1213.gd9a14994de-goog