x86/sev: Use the GHCB protocol when available for SNP CPUID requests
SNP retrieves the majority of CPUID information from the SNP CPUID page. But there are times when that information needs to be supplemented by the hypervisor, for example, obtaining the initial APIC ID of the vCPU from leaf 1. The current implementation uses the MSR protocol to retrieve the data from the hypervisor, even when a GHCB exists. The problem arises when an NMI arrives on return from the VMGEXIT. The NMI will be immediately serviced and may generate a #VC requiring communication with the hypervisor. Since a GHCB exists in this case, it will be used. As part of using the GHCB, the #VC handler will write the GHCB physical address into the GHCB MSR and the #VC will be handled. When the NMI completes, processing resumes at the site of the VMGEXIT which is expecting to read the GHCB MSR and find a CPUID MSR protocol response. Since the NMI handling overwrote the GHCB MSR response, the guest will see an invalid reply from the hypervisor and self-terminate. Fix this problem by using the GHCB when it is available. Any NMI received is properly handled because the GHCB contents are copied into a backup page and restored on NMI exit, thus preserving the active GHCB request or result. [ bp: Touchups. ] Fixes: ee0bfa08 ("x86/compressed/64: Add support for SEV-SNP CPUID table in #VC handlers") Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com> Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de> Cc: <stable@kernel.org> Link: https://lore.kernel.org/r/a5856fa1ebe3879de91a8f6298b6bbd901c61881.1690578565.git.thomas.lendacky@amd.com
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