linux-kernel - Re: [PATCH] KVM: X86: Emulate APERF/MPERF to report actual VCPU frequency

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Message-ID: <CALMp9eTYKQ3LrWKu32mJKPzkWMcN5tGSFmj352TPCSrSp7jGxw@mail.gmail.com>
Date:   Tue, 23 Jun 2020 13:34:19 -0700
From:   Jim Mattson <jmattson@...gle.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Like Xu <like.xu@...ux.intel.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        kvm list <kvm@...r.kernel.org>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Joerg Roedel <joro@...tes.org>, wei.huang2@....com,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Li RongQing <lirongqing@...du.com>,
        Chai Wen <chaiwen@...du.com>, Jia Lina <jialina01@...du.com>
Subject: Re: [PATCH] KVM: X86: Emulate APERF/MPERF to report actual VCPU frequency

On Tue, Jun 23, 2020 at 12:05 PM Sean Christopherson
<sean.j.christopherson@...el.com> wrote:
>
> On Tue, Jun 23, 2020 at 11:39:16AM -0700, Jim Mattson wrote:
> > On Tue, Jun 23, 2020 at 11:29 AM Sean Christopherson
> > <sean.j.christopherson@...el.com> wrote:
> > >
> > > On Tue, Jun 23, 2020 at 02:35:30PM +0800, Like Xu wrote:
> > > > The aperf/mperf are used to report current CPU frequency after 7d5905dc14a
> > > > "x86 / CPU: Always show current CPU frequency in /proc/cpuinfo". But guest
> > > > kernel always reports a fixed VCPU frequency in the /proc/cpuinfo, which
> > > > may confuse users especially when turbo is enabled on the host.
> > > >
> > > > Emulate guest APERF/MPERF capability based their values on the host.
> > > >
> > > > Co-developed-by: Li RongQing <lirongqing@...du.com>
> > > > Signed-off-by: Li RongQing <lirongqing@...du.com>
> > > > Reviewed-by: Chai Wen <chaiwen@...du.com>
> > > > Reviewed-by: Jia Lina <jialina01@...du.com>
> > > > Signed-off-by: Like Xu <like.xu@...ux.intel.com>
> > > > ---
> > >
> > > ...
> > >
> > > > @@ -8312,7 +8376,7 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
> > > >               dm_request_for_irq_injection(vcpu) &&
> > > >               kvm_cpu_accept_dm_intr(vcpu);
> > > >       fastpath_t exit_fastpath;
> > > > -
> > > > +     u64 enter_mperf = 0, enter_aperf = 0, exit_mperf = 0, exit_aperf = 0;
> > > >       bool req_immediate_exit = false;
> > > >
> > > >       if (kvm_request_pending(vcpu)) {
> > > > @@ -8516,8 +8580,17 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
> > > >               vcpu->arch.switch_db_regs &= ~KVM_DEBUGREG_RELOAD;
> > > >       }
> > > >
> > > > +     if (unlikely(vcpu->arch.hwp.hw_coord_fb_cap))
> > > > +             get_host_amperf(&enter_mperf, &enter_aperf);
> > > > +
> > > >       exit_fastpath = kvm_x86_ops.run(vcpu);
> > > >
> > > > +     if (unlikely(vcpu->arch.hwp.hw_coord_fb_cap)) {
> > > > +             get_host_amperf(&exit_mperf, &exit_aperf);
> > > > +             vcpu_update_amperf(vcpu, get_amperf_delta(enter_aperf, exit_aperf),
> > > > +                     get_amperf_delta(enter_mperf, exit_mperf));
> > > > +     }
> > > > +
> > >
> > > Is there an alternative approach that doesn't require 4 RDMSRs on every VMX
> > > round trip?  That's literally more expensive than VM-Enter + VM-Exit
> > > combined.
> > >
> > > E.g. what about adding KVM_X86_DISABLE_EXITS_APERF_MPERF and exposing the
> > > MSRs for read when that capability is enabled?
> >
> > When would you load the hardware MSRs with the guest/host values?
>
> Ugh, I was thinking the MSRs were read-only.

EVen if they were read-only, they should power on to zero, and they
will most likely not be zero when a guest powers on.

> Doesn't this also interact with TSC scaling?

Yes, it should!