[RFC PATCH] x86/sev-es: Include XSS value in GHCB CPUID request

[John Allen <john.allen@amd.com>]

When a guest issues a cpuid instruction for Fn0000000D_x0B
(CetUserOffset), KVM will intercept and need to access the guest
XSS value. For SEV-ES, this is encrypted and needs to be
included in the GHCB to be visible to the hypervisor. The rdmsr
instruction needs to be called directly as the code may be used in early
boot in which case the rdmsr wrappers should be avoided as they are
incompatible with the decompression boot phase.

Signed-off-by: John Allen <john.allen@amd.com>
---
This patch is logically part of the SVM guest shadow stack support series seen
here:
https://lore.kernel.org/all/20221012203910.204793-1-john.allen@amd.com/

Sending this patch separately from the main series as it should apply to the
tip tree as opposed to the kvm tree as this patch is related to guest kernel
support.
---
 arch/x86/kernel/sev-shared.c | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/arch/x86/kernel/sev-shared.c b/arch/x86/kernel/sev-shared.c
index 3a5b0c9c4fcc..34469fac03f0 100644
--- a/arch/x86/kernel/sev-shared.c
+++ b/arch/x86/kernel/sev-shared.c
@@ -887,6 +887,21 @@ 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) {
+		unsigned long lo, hi;
+		u64 xss;
+
+		/*
+		 * Since vc_handle_cpuid may be used during early boot, the
+		 * rdmsr wrappers are incompatible and should not be used.
+		 * Invoke the instruction directly.
+		 */
+		asm volatile("rdmsr" : "=a" (lo), "=d" (hi)
+				    : "c" (MSR_IA32_XSS));
+		xss = (hi << 32) | lo;
+		ghcb_set_xss(ghcb, xss);
+	}
+
 	ret = sev_es_ghcb_hv_call(ghcb, ctxt, SVM_EXIT_CPUID, 0, 0);
 	if (ret != ES_OK)
 		return ret;
-- 
2.34.3