Re: [PATCH v4] KVM: VMX: do not disable interception for MSR_IA32_SPEC_CTRL on eIBRS

[Jim Mattson]

On Thu, Jun 1, 2023 at 1:35 PM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
>
> On Thu, Jun 01, 2023, Jim Mattson wrote:
> > On Thu, Jun 1, 2023 at 12:28 PM Jon Kohler <jon@xxxxxxxxxxx> wrote:
> > > > diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
> > > > index c544602d07a3..454bcbf5b543 100644
> > > > --- a/arch/x86/kvm/x86.h
> > > > +++ b/arch/x86/kvm/x86.h
> > > > @@ -492,7 +492,31 @@ static inline void kvm_machine_check(void)
> > > >
> > > > void kvm_load_guest_xsave_state(struct kvm_vcpu *vcpu);
> > > > void kvm_load_host_xsave_state(struct kvm_vcpu *vcpu);
> > > > +
> > > > int kvm_spec_ctrl_test_value(u64 value);
> > > > +
> > > > +static inline bool kvm_account_msr_spec_ctrl_write(struct kvm_vcpu *vcpu)
> > > > +{
> > > > +     if ((vcpu->stat.exits - vcpu->arch.spec_ctrl_nr_exits_snapshot) < 20)
> >
> > I think you mean 200 here. If it's bad to have more than 1
> > WRMSR(IA32_SPEC_CTRL) VM-exit in 20 VM-exits, then more than 10 such
> > VM-exits in 200 VM-exits represents sustained badness.
>
> No?  The snapshot is updated on every write, i.e. this check is whether or not
> the last wrmsr(SPEC_CTRL) was less than 20 cycles ago.
>
>        if ((vcpu->stat.exits - vcpu->arch.spec_ctrl_nr_exits_snapshot) < 20)
>                vcpu->arch.nr_quick_spec_ctrl_writes++;
>        else
>                vcpu->arch.nr_quick_spec_ctrl_writes = 0;

Okay, then maybe this else clause is too aggressive. Just because we
went 21 VM-exits without seeing a WRMSR(IA32_SPEC_CTRL), we don't want
to clobber all of our history.

>        vcpu->arch.spec_ctrl_nr_exits_snapshot = vcpu->stat.exits;  <= new snapshot
>
>        return vcpu->arch.nr_quick_spec_ctrl_writes >= 10;
>
> > (Although, as Sean noted, these numbers are just placeholders.)
>
> And the logic is very off-the-cuff, e.g. it may be better to have a rolling 200-exit
> window instead of 10 somewhat independent 20-exit windows.