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

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Message-ID: <20250908201750.98824-5-john.allen@amd.com>
Date: Mon, 8 Sep 2025 20:17:49 +0000
From: John Allen <john.allen@....com>
To: <kvm@...r.kernel.org>, <linux-kernel@...r.kernel.org>, <x86@...nel.org>,
	<seanjc@...gle.com>, <pbonzini@...hat.com>, <dave.hansen@...el.com>
CC: <rick.p.edgecombe@...el.com>, <mlevitsk@...hat.com>,
	<weijiang.yang@...el.com>, <chao.gao@...el.com>, <bp@...en8.de>,
	<dave.hansen@...ux.intel.com>, <hpa@...or.com>, <mingo@...hat.com>,
	<tglx@...utronix.de>, <thomas.lendacky@....com>, John Allen
	<john.allen@....com>
Subject: [PATCH v4 4/5] KVM: SVM: Add MSR_IA32_XSS to the GHCB for hypervisor kernel

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@....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