linux-kernel - Re: [PATCH 1/4] KVM: x86: Reject loading KVM if host.PAT[0] != WB

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Message-ID: <CALMp9eQdzZK4ZAyQZXUWff_zuRRdr=ugkujWfFrt9dP8uFcs=Q@mail.gmail.com>
Date:   Fri, 15 Jul 2022 16:06:23 -0700
From:   Jim Mattson <jmattson@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/4] KVM: x86: Reject loading KVM if host.PAT[0] != WB

On Fri, Jul 15, 2022 at 4:02 PM Sean Christopherson <seanjc@...gle.com> wrote:
>
> Reject KVM if entry '0' in the host's IA32_PAT MSR is not programmed to
> writeback (WB) memtype.  KVM subtly relies on IA32_PAT entry '0' to be
> programmed to WB by leaving the PAT bits in shadow paging and NPT SPTEs
> as '0'.  If something other than WB is in PAT[0], at _best_ guests will
> suffer very poor performance, and at worst KVM will crash the system by
> breaking cache-coherency expecations (e.g. using WC for guest memory).
>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
What if someone changes the host's PAT to violate this rule *after*
kvm is loaded?