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

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Date:   Fri, 15 Jul 2022 23:18:03 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Jim Mattson <jmattson@...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, Jim Mattson wrote:
> 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?

Then KVM (and probably many other things in the kernel) is hosed.  The same argument
(that KVM isn't paranoid enough) can likely be made for a number of MSRs and critical
registers.