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

From: Jim Mattson
Date: Fri Jul 15 2022 - 19:06:44 EST

On Fri, Jul 15, 2022 at 4:02 PM Sean Christopherson <seanjc@xxxxxxxxxx> 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@xxxxxxxxxx>
> ---
What if someone changes the host's PAT to violate this rule *after*
kvm is loaded?