linux-kernel - Re: [PATCH] x86/cpu/AMD: Adjust x86_phys_bits to account for reduced PA in SEV-* guests

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Message-ID: <YFJyX/MludiiS+dN@google.com>
Date:   Wed, 17 Mar 2021 14:19:27 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
        Joerg Roedel <joro@...tes.org>,
        Tom Lendacky <thomas.lendacky@....com>,
        Brijesh Singh <brijesh.singh@....com>,
        Peter Gonda <pgonda@...gle.com>
Subject: Re: [PATCH] x86/cpu/AMD: Adjust x86_phys_bits to account for reduced
 PA in SEV-* guests

On Wed, Mar 17, 2021, Sean Christopherson wrote:
> On Wed, Mar 17, 2021, Borislav Petkov wrote:
> > IOW, you have c_bit so your valid address space is [0 .. c_bit-1] no?
> 
> I haven't found anything in the GHCB that dictates that MAXPHYADDR == C_BIT-1,
> or more specifically that MAXPHYADDR == C_BIT - PhysAddrReduction.  E.g. AFAICT,
> a VMM could do C_BIT=47, MAXPHYADDR=36, PhysAddrReduction=0, and that would be
> allowed by the GHCB.
> 
> Forcing "c->x86_phys_bits = c_bit - 1" doesn't seem like it would break anything,
> but it's also technically wrong.

On the other hand, "C_BIT=47, MAXPHYADDR=36, PhysAddrReduction=0" would mean the
C-bit is an illegal PA bit from the guest's perspective.  That's rather
nonsensical, but also not technically disallowed by the APM or GHCB specs.