linux-kernel - Re: [PATCH v2] KVM: SVM: Override default MMIO mask if memory encryption is enabled

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Date:   Tue, 7 Jan 2020 16:04:12 -0800
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Tom Lendacky <thomas.lendacky@....com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Brijesh Singh <brijesh.singh@....com>
Subject: Re: [PATCH v2] KVM: SVM: Override default MMIO mask if memory
 encryption is enabled

On Tue, Jan 07, 2020 at 05:51:51PM -0600, Tom Lendacky wrote:
> On 1/7/20 5:31 PM, Sean Christopherson wrote:
> > AIUI, using phys_bits=48, then the standard scenario is Cbit=47 and some
> > additional bits 46:M are reserved.  Applying that logic to phys_bits=52,
> > then Cbit=51 and bits 50:M are reserved, so there's a collision but it's
> 
> There's no requirement that the C-bit correspond to phys_bits. So, for
> example, you can have C-bit=51 and phys_bits=48 and so 47:M are reserved.

But then using blindly using x86_phys_bits would break if the PA bits
aren't reduced, e.g. C-bit=47 and phys_bits=47. AFAICT, there's no
requirement that there be reduced PA bits when there is a C-bit.  I'm
guessing there aren't plans to ship such CPUs, but I don't see anything
in the APM to prevent such a scenario.

Maybe the least painful approach would be to go with a version of this
patch and add a check that there are indeeded reserved/reduced bits?
Probably with a WARN_ON_ONCE if the check fails.