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

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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
This commit is contained in:
Tom Lendacky 2025-09-10 17:19:28 +02:00 committed by Borislav Petkov (AMD)
parent 26a9f90b61
commit 8d73829b78
1 changed files with 11 additions and 0 deletions
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11
arch/x86/boot/startup/sev-shared.c
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@ -458,6 +458,13 @@ 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;
@ -465,6 +472,10 @@ void do_vc_no_ghcb(struct pt_regs *regs, unsigned long exit_code)
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;