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: <CAMkAt6oLT6-fCYG_t22ZO5K61QjS07HPwgey88p=isbL_ZAMPA@mail.gmail.com>
Date:   Wed, 17 Mar 2021 15:51:37 -0600
From:   Peter Gonda <pgonda@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Borislav Petkov <bp@...en8.de>,
        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>
Subject: Re: [PATCH] x86/cpu/AMD: Adjust x86_phys_bits to account for reduced
 PA in SEV-* guests

On Wed, Mar 17, 2021 at 3:19 PM Sean Christopherson <seanjc@...gle.com> wrote:
>
> 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.

The C-bit location on Rome is 47 but it's 51 on Milan. So we already
have a C-bit that is an illegal PA bit.