x86/startup/sev: Document the CPUID flow in the boot #VC handler

Document the CPUID reading the different SEV guest types do - the SNP
one which relies on the presence of a CPUID table and the SEV-ES one,
which reads the CPUID supplied by the hypervisor.

The intent being to clarify the two back-to-back, similar CPUID
invocations.

No functional changes.

  [ bp: Turn into a proper patch. ]

Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/fbb24767-0e06-d1d6-36e0-1757d98aca66@amd.com

Author: tlendacky

arch/x86/boot/startup/sev-shared.c

@@ -458,13 +458,24 @@ void do_vc_no_ghcb(struct pt_regs *regs, unsigned long exit_code)
 	leaf.fn = fn;
 	leaf.subfn = subfn;
 
+	/*
+	 * If SNP is active, then snp_cpuid() uses the CPUID table to obtain the
+	 * CPUID values (with possible HV interaction during post-processing of
+	 * the values). But if SNP is not active (no CPUID table present), then
+	 * snp_cpuid() returns -EOPNOTSUPP so that an SEV-ES guest can call the
+	 * HV to obtain the CPUID information.
+	 */
 	ret = snp_cpuid(snp_cpuid_hv_msr, NULL, &leaf);
 	if (!ret)
 		goto cpuid_done;
 
 	if (ret != -EOPNOTSUPP)
 		goto fail;
 
+	/*
+	 * This is reached by a SEV-ES guest and needs to invoke the HV for
+	 * the CPUID data.
+	 */
 	if (__sev_cpuid_hv_msr(&leaf))
 		goto fail;