Re: [PATCH RFC 2/7] kvm: x86: Introduce XFD MSRs as passthrough to guest

[Jim Mattson <jmattson@google.com>]

On Mon, May 24, 2021 at 2:44 PM Sean Christopherson <seanjc@google.com> wrote:
>
> On Sun, Feb 07, 2021, Jing Liu wrote:
> > Passthrough both MSRs to let guest access and write without vmexit.
>
> Why?  Except for read-only MSRs, e.g. MSR_CORE_C1_RES, passthrough MSRs are
> costly to support because KVM must context switch the MSR (which, by the by, is
> completely missing from the patch).
>
> In other words, if these MSRs are full RW passthrough, guests with XFD enabled
> will need to load the guest value on entry, save the guest value on exit, and
> load the host value on exit.  That's in the neighborhood of a 40% increase in
> latency for a single VM-Enter/VM-Exit roundtrip (~1500 cycles => >2000 cycles).
>
> I'm not saying these can't be passhthrough, but there needs to be strong
> justification for letting the guest read/write them directly.

If we virtualize XFD, we have to context switch the guest/host values
on VM-entry/VM-exit, don't we? If we don't, we're forced to synthesize
the #NM on any instruction that would access a disabled state
component, and I don't think we have any way of doing that. We could
intercept a guest WRMSR to these MSRs, but it sounds like the guest
can still implicitly write to IA32_XFD_ERR, if we allow it to have a
non-zero IA32_XFD.

Perhaps the answer is "don't virtualize XFD."