[PATCH v4 4/5] KVM: SVM: Add MSR_IA32_XSS to the GHCB for hypervisor kernel

John Allen
2025-09-08T20:18:48Z

When a guest issues a cpuid instruction for Fn0000000D_x0B_{x00,x01}, KVM will
be intercepting the CPUID instruction and will need to access the guest
MSR_IA32_XSS value. For SEV-ES, the XSS value is encrypted and needs to be
included in the GHCB to be visible to the hypervisor.

Signed-off-by: John Allen <john.allen@amd.com>
---
v2:
- Omit passing through XSS as this has already been properly
implemented in a26b7cd22546 ("KVM: SEV: Do not intercept
accesses to MSR_IA32_XSS for SEV-ES guests")
v3:
- Move guest kernel GHCB_ACCESSORS definition to new series.
v4:
- Change logic structure to be more intuitive.
---
arch/x86/kvm/svm/sev.c | 5 +++++
arch/x86/kvm/svm/svm.h | 1 +
2 files changed, 6 insertions(+)

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index f4381878a9e5..33c42dd853b3 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -3310,6 +3310,11 @@ static void sev_es_sync_from_ghcb(struct vcpu_svm *svm)
vcpu->arch.cpuid_dynamic_bits_dirty = true;
}

+ if (kvm_ghcb_xss_is_valid(svm)) {
+ vcpu->arch.ia32_xss = ghcb_get_xss(ghcb);
+ vcpu->arch.cpuid_dynamic_bits_dirty = true;
+ }
+
/* Copy the GHCB exit information into the VMCB fields */
exit_code = ghcb_get_sw_exit_code(ghcb);
control->exit_code = lower_32_bits(exit_code);
diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
index 3c7f208b7935..552c58b050f1 100644
--- a/arch/x86/kvm/svm/svm.h
+++ b/arch/x86/kvm/svm/svm.h
@@ -935,5 +935,6 @@ DEFINE_KVM_GHCB_ACCESSORS(sw_exit_info_1)
DEFINE_KVM_GHCB_ACCESSORS(sw_exit_info_2)
DEFINE_KVM_GHCB_ACCESSORS(sw_scratch)
DEFINE_KVM_GHCB_ACCESSORS(xcr0)
+DEFINE_KVM_GHCB_ACCESSORS(xss)

#endif
--
2.47.3