linux-kernel - Re: [PATCH 07/12] KVM: x86: SEV: Treat C-bit as legal GPA bit regardless of vCPU mode

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Message-ID: <YBw0a5fFvtOrDwOR@google.com>
Date:   Thu, 4 Feb 2021 09:52:43 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
Cc:     "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "jmattson@...gle.com" <jmattson@...gle.com>,
        "joro@...tes.org" <joro@...tes.org>,
        "vkuznets@...hat.com" <vkuznets@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "thomas.lendacky@....com" <thomas.lendacky@....com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "wanpengli@...cent.com" <wanpengli@...cent.com>,
        "brijesh.singh@....com" <brijesh.singh@....com>
Subject: Re: [PATCH 07/12] KVM: x86: SEV: Treat C-bit as legal GPA bit
 regardless of vCPU mode

On Thu, Feb 04, 2021, Edgecombe, Rick P wrote:
> On Thu, 2021-02-04 at 11:34 +0100, Paolo Bonzini wrote:
> > On 04/02/21 03:19, Sean Christopherson wrote:
> > > Ah, took me a few minutes, but I see what you're saying.  LAM will
> > > introduce
> > > bits that are repurposed for CR3, but not generic GPAs.  And, the
> > > behavior is
> > > based on CPU support, so it'd make sense to have a mask cached in
> > > vcpu->arch
> > > as opposed to constantly generating it on the fly.
> > > 
> > > Definitely agree that having a separate cr3_lm_rsvd_bits or
> > > whatever is the
> > > right way to go when LAM comes along.  Not sure it's worth keeping
> > > a duplicate
> > > field in the meantime, though it would avoid a small amount of
> > > thrash.
> > 
> > We don't even know if the cr3_lm_rsvd_bits would be a field in 
> > vcpu->arch, or rather computed on the fly.  So renaming the field in 
> > vcpu->arch seems like the simplest thing to do now.
> 
> Fair enough. But just to clarify, I meant that I thought the code would
> be more confusing to use illegal gpa bit checks for checking cr3. It
> seems they are only incidentally the same value.

Hmm, yeah, bits 63:52 are incidental.  Bits 52:M are not, though.  If/when we
need to special case CR3, I would like to take a similar approach to
__reset_rsvds_bits_mask(), where the high reserved bits start from
reserved_gpa_bits and mask off the bits that can't be encoded into a PxE.

> Alternatively there could be something like a is_rsvd_cr3_bits() helper that
> just uses reserved_gpa_bits for now. Probably put the comment in the wrong
> place.  It's a minor point in any case.

That thought crossed my mind, too.  Maybe kvm_vcpu_is_illegal_cr3() to match
the gpa helpers?