Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Thu, 1 Jun 2023 13:35:51 -0700
In-Reply-To: <CALMp9eQ247GCxHnn3VwFatKEswWq9cMaoZCOivC-OQ_asvFHZQ@mail.gmail.com>
Mime-Version: 1.0
References: <9d37ab07-97c6-5245-6939-9c1090b4b3a9@redhat.com>
 <ECE9A3B4-2CF3-452A-838A-99BE075D5E68@nutanix.com> <3cb96907-0d58-ba60-ed0e-6c17c69bd0f8@redhat.com>
 <CALMp9eQNNCwUbPQGBfHzWnTAEJeRO-fjQAFxb9101SChe9F5rg@mail.gmail.com>
 <623EC08D-A755-4520-B9BF-42B0E72570C1@nutanix.com> <CALMp9eQ17+XRpxJjMnmvPnKOC1VP1P=mU-KykoOzYZsgtGN8sQ@mail.gmail.com>
 <658D3EF0-B2D3-4492-A2A1-FC84A58B201D@nutanix.com> <ZHjYsKVBFLsOmHcF@google.com>
 <BF7121B7-B2AE-4391-9D1B-D944B5BC44D0@nutanix.com> <CALMp9eQ247GCxHnn3VwFatKEswWq9cMaoZCOivC-OQ_asvFHZQ@mail.gmail.com>
Message-ID: <ZHkBJ+RdPYIZjolX@google.com>
Subject: Re: [PATCH v4] KVM: VMX: do not disable interception for
 MSR_IA32_SPEC_CTRL on eIBRS
From:   Sean Christopherson <seanjc@google.com>
To:     Jim Mattson <jmattson@google.com>
Cc:     Jon Kohler <jon@nutanix.com>, Waiman Long <longman@redhat.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        "x86@kernel.org" <x86@kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Josh Poimboeuf <jpoimboe@kernel.org>,
        Kees Cook <keescook@chromium.org>,
        Andrea Arcangeli <aarcange@redhat.com>,
        "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>
Content-Type: text/plain; charset="utf-8"
Content-Transfer-Encoding: quoted-printable
Precedence: bulk

On Thu, Jun 01, 2023, Jim Mattson wrote:
> On Thu, Jun 1, 2023 at 12:28=E2=80=AFPM Jon Kohler <jon@nutanix.com> wrot=
e:
> > > 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)
>=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. =20

       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 =3D 0;

       vcpu->arch.spec_ctrl_nr_exits_snapshot =3D vcpu->stat.exits;  <=3D n=
ew snapshot

       return vcpu->arch.nr_quick_spec_ctrl_writes >=3D 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.