linux-kernel - Re: [PATCH v6 08/42] x86/sev-es: initialize sev

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Message-ID: <YXGlPf5OTPzp09qr@zn.tnic>
Date:   Thu, 21 Oct 2021 19:37:01 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     "Dr. David Alan Gilbert" <dgilbert@...hat.com>
Cc:     Michael Roth <michael.roth@....com>,
        Brijesh Singh <brijesh.singh@....com>, 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>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Andi Kleen <ak@...ux.intel.com>, tony.luck@...el.com,
        marcorr@...gle.com, sathyanarayanan.kuppuswamy@...ux.intel.com
Subject: Re: [PATCH v6 08/42] x86/sev-es: initialize sev_status/features
 within #VC handler

On Thu, Oct 21, 2021 at 06:12:53PM +0100, Dr. David Alan Gilbert wrote:
> OK, so that bit is 8...21 Eax ext2eax bit 6 page 1-109
> 
> then 2.1.5.3 CPUID policy enforcement shows 8...21 EAX as
> 'bitmask'
> 'bits set in the GuestVal must also be set in HostVal.
> This is often applied to feature fields where each bit indicates
> support for a feature'
> 
> So that's right isn't it?

Yap, AFAIRC, it would fail the check if:

(GuestVal & HostVal) != GuestVal

and GuestVal is "the CPUID result value created by the hypervisor that
it wants to give to the guest". Let's say it clears bit 6 there.

Then HostVal comes in which is "the actual CPUID result value specified
in this PPR" and there the guest catches the HV lying its *ss off.

:-)

-- 
Regards/Gruss,
    Boris.

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