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Date:   Thu, 21 Oct 2021 19:37:01 +0200
From:   Borislav Petkov <bp@alien8.de>
To:     "Dr. David Alan Gilbert" <dgilbert@redhat.com>
Cc:     Michael Roth <michael.roth@amd.com>,
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Subject: Re: [PATCH v6 08/42] x86/sev-es: initialize sev_status/features
 within #VC handler
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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