linux-kernel - Re: [PATCH v6 14/42] x86/sev: Register GHCB memory when SEV-SNP is active

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Message-ID: <YYFs+5UUMfyDgh/a@zn.tnic>
Date:   Tue, 2 Nov 2021 17:53:15 +0100
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,
        linux-efi@...r.kernel.org, platform-driver-x86@...r.kernel.org,
        linux-coco@...ts.linux.dev, linux-mm@...ck.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Joerg Roedel <jroedel@...e.de>,
        Tom Lendacky <thomas.lendacky@....com>,
        "H. Peter Anvin" <hpa@...or.com>, Ard Biesheuvel <ardb@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Jim Mattson <jmattson@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Sergio Lopez <slp@...hat.com>, Peter Gonda <pgonda@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        David Rientjes <rientjes@...gle.com>,
        Dov Murik <dovmurik@...ux.ibm.com>,
        Tobin Feldman-Fitzthum <tobin@....com>,
        Michael Roth <michael.roth@....com>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Andi Kleen <ak@...ux.intel.com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        tony.luck@...el.com, marcorr@...gle.com,
        sathyanarayanan.kuppuswamy@...ux.intel.com
Subject: Re: [PATCH v6 14/42] x86/sev: Register GHCB memory when SEV-SNP is
 active

On Fri, Oct 08, 2021 at 01:04:25PM -0500, Brijesh Singh wrote:
> +	/* SEV-SNP guest requires that GHCB must be registered. */
> +	if (cc_platform_has(CC_ATTR_SEV_SNP))
> +		snp_register_ghcb(data, __pa(ghcb));

This looks like more of that "let's register a GHCB at the time the
first #VC fires".

And there already is setup_ghcb() which is called in the #VC handler.
And that thing registers a GHCB GPA.

But then you have to do it here again.

I think this should be changed together with the CPUID page detection
stuff we talked about earlier, where, after you've established that this
is an SNP guest, you call setup_ghcb() *once* and after that you have
everything set up, including the GHCB GPA. And then the #VC exceptions
can come.

Right?

Or is there a chicken-and-an-egg issue here which I'm not thinking
about?

-- 
Regards/Gruss,
    Boris.

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