linux-kernel - Re: [PATCH Part1 RFC v2 02/20] x86/sev: Save the negotiated GHCB version

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Date:   Tue, 11 May 2021 20:41:26 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Brijesh Singh <brijesh.singh@....com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        tglx@...utronix.de, jroedel@...e.de, thomas.lendacky@....com,
        pbonzini@...hat.com, mingo@...hat.com, dave.hansen@...el.com,
        rientjes@...gle.com, seanjc@...gle.com, peterz@...radead.org,
        hpa@...or.com, tony.luck@...el.com
Subject: Re: [PATCH Part1 RFC v2 02/20] x86/sev: Save the negotiated GHCB
 version

On Tue, May 11, 2021 at 01:29:00PM -0500, Brijesh Singh wrote:
> I ignored it, because I thought I still need to initialize the value to
> zero because it does not go into .bss section which gets initialized to
> zero by kernel on boot.
> 
> I guess I was wrong, maybe compiler or kernel build ensures that
> ghcb_version variable to be init'ed to zero because its marked static ?

Yes.

If in doubt, always look at the generated asm:

make arch/x86/kernel/sev.s

You can see the .zero 2 gas directive there, where the variable is
defined.

> In patch #4, you will see that I increase the GHCB max protocol version
> to 2, and then min_t() will choose the lower value. I didn't combine
> version bump and caching into same patch because I felt I will be asked
> to break them into two logical patches.

Hmm, what would be the reasoning to keep the version bump in a separate
patch?

-- 
Regards/Gruss,
    Boris.

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