[PATCH] arch/x86: Set XSS while handling #VC intercept for CPUID

[Jinank Jain <jinankjain@linux.microsoft.com>]

According to [1], while handling the #VC intercept for CPUID leaf
0x0000_000D, we need to supply the value of XSS in the GHCB page. If
this value is not provided then a spec compliant hypervisor can fail the
GHCB request and kill the guest.

[1] https://www.amd.com/system/files/TechDocs/56421-guest-hypervisor-communication-block-standardization.pdf

Signed-off-by: Jinank Jain <jinankjain@linux.microsoft.com>
---
 arch/x86/include/asm/svm.h   | 1 +
 arch/x86/kernel/sev-shared.c | 3 +++
 2 files changed, 4 insertions(+)

diff --git a/arch/x86/include/asm/svm.h b/arch/x86/include/asm/svm.h
index 19bf955b67e0..c2f670f7cb47 100644
--- a/arch/x86/include/asm/svm.h
+++ b/arch/x86/include/asm/svm.h
@@ -678,5 +678,6 @@ 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
diff --git a/arch/x86/kernel/sev-shared.c b/arch/x86/kernel/sev-shared.c
index 2eabccde94fb..92350a24848c 100644
--- a/arch/x86/kernel/sev-shared.c
+++ b/arch/x86/kernel/sev-shared.c
@@ -880,6 +880,9 @@ static enum es_result vc_handle_cpuid(struct ghcb *ghcb,
 	if (snp_cpuid_ret != -EOPNOTSUPP)
 		return ES_VMM_ERROR;
 
+	if (regs->ax == 0xD && regs->cx == 0x1)
+		ghcb_set_xss(ghcb, 0);
+
 	ghcb_set_rax(ghcb, regs->ax);
 	ghcb_set_rcx(ghcb, regs->cx);
 
-- 
2.34.1