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Date:   Tue, 2 Nov 2021 17:53:15 +0100
From:   Borislav Petkov <bp@alien8.de>
To:     Brijesh Singh <brijesh.singh@amd.com>
Cc:     x86@kernel.org, linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        linux-efi@vger.kernel.org, platform-driver-x86@vger.kernel.org,
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        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Joerg Roedel <jroedel@suse.de>,
        Tom Lendacky <thomas.lendacky@amd.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Ard Biesheuvel <ardb@kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Sean Christopherson <seanjc@google.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Jim Mattson <jmattson@google.com>,
        Andy Lutomirski <luto@kernel.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
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        Dov Murik <dovmurik@linux.ibm.com>,
        Tobin Feldman-Fitzthum <tobin@ibm.com>,
        Michael Roth <michael.roth@amd.com>,
        Vlastimil Babka <vbabka@suse.cz>,
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        Andi Kleen <ak@linux.intel.com>,
        "Dr . David Alan Gilbert" <dgilbert@redhat.com>,
        tony.luck@intel.com, marcorr@google.com,
        sathyanarayanan.kuppuswamy@linux.intel.com
Subject: Re: [PATCH v6 14/42] x86/sev: Register GHCB memory when SEV-SNP is
 active
Message-ID: <YYFs+5UUMfyDgh/a@zn.tnic>
References: <20211008180453.462291-1-brijesh.singh@amd.com>
 <20211008180453.462291-15-brijesh.singh@amd.com>
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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