x86/sev: Provide SVSM discovery support

The SVSM specification documents an alternative method of discovery for
the SVSM using a reserved CPUID bit and a reserved MSR. This is intended
for guest components that do not have access to the secrets page in
order to be able to call the SVSM (e.g. UEFI runtime services).

For the MSR support, a new reserved MSR 0xc001f000 has been defined. A #VC
should be generated when accessing this MSR. The #VC handler is expected
to ignore writes to this MSR and return the physical calling area address
(CAA) on reads of this MSR.

While the CPUID leaf is updated, allowing the creation of a CPU feature,
the code will continue to use the VMPL level as an indication of the
presence of an SVSM. This is because the SVSM can be called well before
the CPU feature is in place and a non-zero VMPL requires that an SVSM be
present.

Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/4f93f10a2ff3e9f368fd64a5920d51bf38d0c19e.1717600736.git.thomas.lendacky@amd.com

arch/x86/include/asm/cpufeatures.h

@@ -446,6 +446,7 @@
 #define X86_FEATURE_V_TSC_AUX		(19*32+ 9) /* "" Virtual TSC_AUX */
 #define X86_FEATURE_SME_COHERENT	(19*32+10) /* "" AMD hardware-enforced cache coherency */
 #define X86_FEATURE_DEBUG_SWAP		(19*32+14) /* AMD SEV-ES full debug state swap support */
+#define X86_FEATURE_SVSM		(19*32+28) /* SVSM present */
 
 /* AMD-defined Extended Feature 2 EAX, CPUID level 0x80000021 (EAX), word 20 */
 #define X86_FEATURE_NO_NESTED_DATA_BP	(20*32+ 0) /* "" No Nested Data Breakpoints */

arch/x86/include/asm/msr-index.h

@@ -660,6 +660,8 @@
 #define MSR_AMD64_RMP_BASE		0xc0010132
 #define MSR_AMD64_RMP_END		0xc0010133
 
+#define MSR_SVSM_CAA			0xc001f000
+
 /* AMD Collaborative Processor Performance Control MSRs */
 #define MSR_AMD_CPPC_CAP1		0xc00102b0
 #define MSR_AMD_CPPC_ENABLE		0xc00102b1

arch/x86/kernel/sev-shared.c

@@ -1648,6 +1648,8 @@ static enum es_result vc_check_opcode_bytes(struct es_em_ctxt *ctxt,
 static bool __head svsm_setup_ca(const struct cc_blob_sev_info *cc_info)
 {
 	struct snp_secrets_page *secrets_page;
+	struct snp_cpuid_table *cpuid_table;
+	unsigned int i;
 	u64 caa;
 
 	BUILD_BUG_ON(sizeof(*secrets_page) != PAGE_SIZE);
@@ -1701,5 +1703,14 @@ static bool __head svsm_setup_ca(const struct cc_blob_sev_info *cc_info)
 	RIP_REL_REF(boot_svsm_caa) = (struct svsm_ca *)caa;
 	RIP_REL_REF(boot_svsm_caa_pa) = caa;
 
+	/* Advertise the SVSM presence via CPUID. */
+	cpuid_table = (struct snp_cpuid_table *)snp_cpuid_get_table();
+	for (i = 0; i < cpuid_table->count; i++) {
+		struct snp_cpuid_fn *fn = &cpuid_table->fn[i];
+
+		if (fn->eax_in == 0x8000001f)
+			fn->eax |= BIT(28);
+	}
+
 	return true;
 }

arch/x86/kernel/sev.c

@@ -1340,6 +1340,17 @@ static enum es_result vc_handle_msr(struct ghcb *ghcb, struct es_em_ctxt *ctxt)
 	/* Is it a WRMSR? */
 	exit_info_1 = (ctxt->insn.opcode.bytes[1] == 0x30) ? 1 : 0;
 
+	if (regs->cx == MSR_SVSM_CAA) {
+		/* Writes to the SVSM CAA msr are ignored */
+		if (exit_info_1)
+			return ES_OK;
+
+		regs->ax = lower_32_bits(this_cpu_read(svsm_caa_pa));
+		regs->dx = upper_32_bits(this_cpu_read(svsm_caa_pa));
+
+		return ES_OK;
+	}
+
 	ghcb_set_rcx(ghcb, regs->cx);
 	if (exit_info_1) {
 		ghcb_set_rax(ghcb, regs->ax);