linux-kernel - [PATCH v2 2/2] x86/sev-es: Include XSS value in GHCB CPUID request

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Message-ID: <20250908202034.98854-3-john.allen@amd.com>
Date: Mon, 8 Sep 2025 20:20:34 +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 v2 2/2] x86/sev-es: Include XSS value in GHCB CPUID request

When a guest issues a cpuid instruction for Fn0000000D_{x00,x01}, the
hypervisor will be intercepting the CPUID instruction and will need to access
the guest XSS value. For SEV-ES, the XSS value is encrypted and needs to be
included in the GHCB to be visible to the hypervisor.

Reviewed-by: Tom Lendacky <thomas.lendacky@....com>
Signed-off-by: John Allen <john.allen@....com>
---
 arch/x86/coco/sev/vc-shared.c | 11 +++++++++++
 arch/x86/include/asm/svm.h    |  1 +
 2 files changed, 12 insertions(+)

diff --git a/arch/x86/coco/sev/vc-shared.c b/arch/x86/coco/sev/vc-shared.c
index 2c0ab0fdc060..079fffdb12c0 100644
--- a/arch/x86/coco/sev/vc-shared.c
+++ b/arch/x86/coco/sev/vc-shared.c
@@ -1,5 +1,9 @@
 // SPDX-License-Identifier: GPL-2.0
 
+#ifndef __BOOT_COMPRESSED
+#define has_cpuflag(f)                  boot_cpu_has(f)
+#endif
+
 static enum es_result vc_check_opcode_bytes(struct es_em_ctxt *ctxt,
 					    unsigned long exit_code)
 {
@@ -452,6 +456,13 @@ static enum es_result vc_handle_cpuid(struct ghcb *ghcb,
 		/* xgetbv will cause #GP - use reset value for xcr0 */
 		ghcb_set_xcr0(ghcb, 1);
 
+	if (has_cpuflag(X86_FEATURE_SHSTK) && regs->ax == 0xd && regs->cx <= 1) {
+		struct msr m;
+
+		raw_rdmsr(MSR_IA32_XSS, &m);
+		ghcb_set_xss(ghcb, m.q);
+	}
+
 	ret = sev_es_ghcb_hv_call(ghcb, ctxt, SVM_EXIT_CPUID, 0, 0);
 	if (ret != ES_OK)
 		return ret;
diff --git a/arch/x86/include/asm/svm.h b/arch/x86/include/asm/svm.h
index 17f6c3fedeee..0581c477d466 100644
--- a/arch/x86/include/asm/svm.h
+++ b/arch/x86/include/asm/svm.h
@@ -701,5 +701,6 @@ DEFINE_GHCB_ACCESSORS(sw_exit_info_1)
 DEFINE_GHCB_ACCESSORS(sw_exit_info_2)
 DEFINE_GHCB_ACCESSORS(sw_scratch)
 DEFINE_GHCB_ACCESSORS(xcr0)
+DEFINE_GHCB_ACCESSORS(xss)
 
 #endif
-- 
2.47.3