linux-kernel - Re: [PATCH v8 06/40] x86/sev: Check SEV-SNP features support

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Message-ID: <YbtwTne5BjERZV1r@zn.tnic>
Date:   Thu, 16 Dec 2021 17:58:54 +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 v8 06/40] x86/sev: Check SEV-SNP features support

On Thu, Dec 16, 2021 at 10:28:45AM -0600, Brijesh Singh wrote:
> A good question; the GHCB page is needed only at the time of #VC.  If the
> second stage VC handler is not called after the sev_enable() during the
> decompression stage, setting up the GHC page in sev_enable() is a waste.

It would be a waste if no #VC would fire. But we set up a #VC handler so
we might just as well set up the GHCB for it too.

> But in practice, the second stage VC handler will be called during
> decompression. It also brings a similar question for the kernel
> proper, should we do the same over there?

I'd think so, yes.
 
> Jorge did the initial ES support and may have other reasons he chose to set
> up GHCB page in the handler. I was trying to avoid the flow change. We can
> do this as a pre or post-SNP patch; let me know your thoughts?

You can do a separate patch only with that change and if it causes
trouble, we can always debug/delay it.

Thx.

-- 
Regards/Gruss,
    Boris.

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