[PATCH kernel v3] x86/compressed/64: reduce #VC nesting for intercepted CPUID for SEV-SNP guest

[Alexey Kardashevskiy]

For certain intercepts an SNP guest uses the GHCB protocol to talk to
the hypervisor from the #VC handler. The protocol requires a shared page so
there is one per vCPU. In case NMI arrives in a middle of #VC or the NMI
handler triggers a #VC, there is another "backup" GHCB page which stores
the content of the first one while SVM_VMGEXIT_NMI_COMPLETE is sent.
The vc_raw_handle_exception() handler manages main and backup GHCB pages
via __sev_get_ghcb/__sev_put_ghcb.

This works fine for #VC and occasional NMIs but not so fine when the #VC
handler causes intercept + another #VC. If NMI arrives during
the second #VC, there are no more pages for SVM_VMGEXIT_NMI_COMPLETE.
The problem place is the #VC CPUID handler which reads an MSR which
triggers another #VC and if "perf" was running, panic happens:

Kernel panic - not syncing: Unable to handle #VC exception! GHCB and Backup GHCB are already in use

Add a helper similar to native_read_msr_safe() for making a direct hypercall
in the SEV-ES environment. Use the new helper instead of the raw "rdmsr" to
avoid the extra #VC event.

Fixes: ee0bfa08a345 ("x86/compressed/64: Add support for SEV-SNP CPUID table in #VC handlers")
Signed-off-by: Alexey Kardashevskiy <aik@xxxxxxx>
---

Based on:
https://git.kernel.org/pub/scm/linux/kernel/git/bp/bp.git/log/?h=tip-x86-urgent
which top at the time was:
62d5e970d022 "x86/sev: Change npages to unsigned long in snp_accept_memory()"

---
Changes:
v3:
* made it a function, mimic native_read_msr_safe() which 1) returns value 2) returns an error
* removed debug backtraces the commit log as these were added for debugging and never
appear with actual kernels


v2:
* de-uglify by defining rdmsr_safe_GHCB()
---
 arch/x86/kernel/sev-shared.c | 27 +++++++++++++++++---
 1 file changed, 23 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kernel/sev-shared.c b/arch/x86/kernel/sev-shared.c
index dcf325b7b022..494d92a71986 100644
--- a/arch/x86/kernel/sev-shared.c
+++ b/arch/x86/kernel/sev-shared.c
@@ -241,6 +241,25 @@ static enum es_result sev_es_ghcb_hv_call(struct ghcb *ghcb,
 	return verify_exception_info(ghcb, ctxt);
 }
 
+
+/* Paravirt SEV-ES rdmsr which avoids extra #VC event */
+static unsigned long long ghcb_prot_read_msr(unsigned int msr, struct ghcb *ghcb,
+					     struct es_em_ctxt *ctxt, int *err)
+{
+	unsigned long long ret = 0;
+
+	ghcb_set_rcx(ghcb, msr);
+
+	*err = sev_es_ghcb_hv_call(ghcb, ctxt, SVM_EXIT_MSR, 0, 0);
+	if (*err == ES_OK)
+		ret = (ghcb->save.rdx << 32) | ghcb->save.rax;
+
+	/* Invalidate qwords for likely another following GHCB call */
+	vc_ghcb_invalidate(ghcb);
+
+	return ret;
+}
+
 static int __sev_cpuid_hv(u32 fn, int reg_idx, u32 *reg)
 {
 	u64 val;
@@ -477,11 +496,11 @@ static int snp_cpuid_postprocess(struct ghcb *ghcb, struct es_em_ctxt *ctxt,
 		if (leaf->subfn == 1) {
 			/* Get XSS value if XSAVES is enabled. */
 			if (leaf->eax & BIT(3)) {
-				unsigned long lo, hi;
+				int err = 0;
 
-				asm volatile("rdmsr" : "=a" (lo), "=d" (hi)
-						     : "c" (MSR_IA32_XSS));
-				xss = (hi << 32) | lo;
+				xss = ghcb_prot_read_msr(MSR_IA32_XSS, ghcb, ctxt, &err);
+				if (err != ES_OK)
+					return -EINVAL;
 			}
 
 			/*
-- 
2.41.0