linux-kernel - Re: [PATCH v4] KVM: VMX: do not disable interception for MSR_IA32_SPEC

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Date:   Thu, 1 Jun 2023 13:35:51 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Jim Mattson <jmattson@...gle.com>
Cc:     Jon Kohler <jon@...anix.com>, Waiman Long <longman@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: [PATCH v4] KVM: VMX: do not disable interception for
 MSR_IA32_SPEC_CTRL on eIBRS

On Thu, Jun 01, 2023, Jim Mattson wrote:
> On Thu, Jun 1, 2023 at 12:28 PM Jon Kohler <jon@...anix.com> 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;

       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.