linux-kernel - Re: [RFT PATCH v3 12/21] x86/sev: Unify SEV-SNP hypervisor feature check

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Message-ID: <CAMj1kXEeJMV5AX9NjLYEwukvpKrDQCvzRyEivj43P7TRH3qFVg@mail.gmail.com>
Date: Fri, 30 May 2025 18:12:43 +0200
From: Ard Biesheuvel <ardb@...nel.org>
To: Borislav Petkov <bp@...en8.de>
Cc: Ard Biesheuvel <ardb+git@...gle.com>, linux-kernel@...r.kernel.org, 
	linux-efi@...r.kernel.org, x86@...nel.org, Ingo Molnar <mingo@...nel.org>, 
	Dionna Amalie Glaze <dionnaglaze@...gle.com>, Kevin Loughlin <kevinloughlin@...gle.com>, 
	Tom Lendacky <thomas.lendacky@....com>
Subject: Re: [RFT PATCH v3 12/21] x86/sev: Unify SEV-SNP hypervisor feature check

On Fri, 30 May 2025 at 18:08, Borislav Petkov <bp@...en8.de> wrote:
>
> On Fri, May 30, 2025 at 04:28:52PM +0200, Ard Biesheuvel wrote:
> > > > +u64 __head snp_check_hv_features(void)
> > > > +{
> > > > +     /*
> > > > +      * SNP is supported in v2 of the GHCB spec which mandates support for HV
> > > > +      * features.
> > > > +      */
> >
> > ... get_hv_features() is only when SEV-SNP has already been detected.
>
> Hmm, I see
>
> void sev_enable(struct boot_params *bp)
> {
>         ...
>
>         /*
>          * Setup/preliminary detection of SNP. This will be sanity-checked
>          * against CPUID/MSR values later.
>          */
>         snp = early_snp_init(bp);
>
>         ...
>
>         snp_check_hv_features();
>
>         if (snp && !(sev_status & MSR_AMD64_SEV_SNP_ENABLED))
>
> This is called here without checking the snp boolean.
>
> And without checking the version it is fragile anyway. Why do you even need to
> remove the version check?
>

Because the assignment is moved out of the startup code later.