linux-kernel - Re: [PATCH v10 04/50] x86/cpufeatures: Add SEV-SNP CPU feature

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Date:   Wed, 13 Dec 2023 16:41:07 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Michael Roth <michael.roth@....com>, kvm@...r.kernel.org,
        linux-coco@...ts.linux.dev, linux-mm@...ck.org,
        linux-crypto@...r.kernel.org, x86@...nel.org,
        linux-kernel@...r.kernel.org, tglx@...utronix.de, mingo@...hat.com,
        jroedel@...e.de, thomas.lendacky@....com, hpa@...or.com,
        ardb@...nel.org, seanjc@...gle.com, vkuznets@...hat.com,
        jmattson@...gle.com, luto@...nel.org, dave.hansen@...ux.intel.com,
        slp@...hat.com, pgonda@...gle.com, peterz@...radead.org,
        srinivas.pandruvada@...ux.intel.com, rientjes@...gle.com,
        dovmurik@...ux.ibm.com, tobin@....com, vbabka@...e.cz,
        kirill@...temov.name, ak@...ux.intel.com, tony.luck@...el.com,
        marcorr@...gle.com, sathyanarayanan.kuppuswamy@...ux.intel.com,
        alpergun@...gle.com, jarkko@...nel.org, ashish.kalra@....com,
        nikunj.dadhania@....com, pankaj.gupta@....com,
        liam.merwick@...cle.com, zhi.a.wang@...el.com,
        Brijesh Singh <brijesh.singh@....com>,
        Jarkko Sakkinen <jarkko@...fian.com>
Subject: Re: [PATCH v10 04/50] x86/cpufeatures: Add SEV-SNP CPU feature

On Wed, Dec 13, 2023 at 03:18:17PM +0100, Paolo Bonzini wrote:
> Surely we can agree that cpu_feature_enabled(X86_FEATURE_SEV_SNP) has nothing
> to do with SEV-SNP host patches being present? 

It does - we're sanitizing the meaning of a CPUID flag present in
/proc/cpuinfo, see here:

https://git.kernel.org/tip/79c603ee43b2674fba0257803bab265147821955

> And that therefore retpolines are preferred even without any SEV-SNP
> support in KVM?

No, automatic IBRS should be disabled when SNP is enabled. Not CPUID
present - enabled. We clear that bit on a couple of occasions in the SNP
host patchset if we determine that SNP host support is not possible so
4/50 needs to go together with the rest to mean something.

> And can we agree that "Acked-by" means "feel free and take it if you wish,

I can see how it can mean that and I'm sorry for the misunderstanding
I caused. Two things here:

* I acked it because I did a lengthly digging internally on whether
disabling AIBRS makes sense on SNP and this was a note more to myself to
say, yes, that's a good change.

* If I wanted for you to pick it up, I would've acked 4/50 too. Which
I haven't.

> I'm asking because I'm not sure if we agree on these two things, but they
> really seem basic to me?

I think KVM and x86 maintainers should sit down and discuss who picks up
what and through which tree so that there's no more confusion in the
future. It seems things need discussion...

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette