linux-kernel - Re: [RFC v2 2/2] x86/kvmclock: Introduce kvm-hostclock clocksource.

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Date:   Tue, 15 Oct 2019 17:39:55 +0900
From:   Suleiman Souhlal <suleiman@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     rkrcmar@...hat.com, Thomas Gleixner <tglx@...utronix.de>,
        John Stultz <john.stultz@...aro.org>, sboyd@...nel.org,
        Linux Kernel <linux-kernel@...r.kernel.org>,
        kvm@...r.kernel.org, Suleiman Souhlal <ssouhlal@...ebsd.org>,
        tfiga@...omium.org, Vitaly Kuznetsov <vkuznets@...hat.com>
Subject: Re: [RFC v2 2/2] x86/kvmclock: Introduce kvm-hostclock clocksource.

On Thu, Oct 10, 2019 at 7:55 PM Paolo Bonzini <pbonzini@...hat.com> wrote:
>
> On 10/10/19 09:30, Suleiman Souhlal wrote:
> > +kvm_hostclock_enable(struct clocksource *cs)
> > +{
> > +     pv_timekeeper_enabled = 1;
> > +
> > +     old_vclock_mode = kvm_clock.archdata.vclock_mode;
> > +     kvm_clock.archdata.vclock_mode = VCLOCK_TSC;
> > +     return 0;
> > +}
> > +
> > +static void
> > +kvm_hostclock_disable(struct clocksource *cs)
> > +{
> > +     pv_timekeeper_enabled = 0;
> > +     kvm_clock.archdata.vclock_mode = old_vclock_mode;
> > +}
> > +
>
> Why do you poke at kvm_clock?  Instead you should add
>
>         .archdata               = { .vclock_mode = VCLOCK_TSC },
>
> to the kvm_hostclock declaration.

Yeah, what I was doing does not make sense.

> Please also check that the invariant TSC CPUID bit
> CPUID[0x80000007].EDX[8] is set before enabling this feature.

Will do.

>
> Paolo
>
> > +     pvtk = &pv_timekeeper;
> > +     do {
> > +             gen = pvtk_read_begin(pvtk);
> > +             if (!(pv_timekeeper.flags & PVCLOCK_TIMEKEEPER_ENABLED))
> > +                     return;
> > +
> > +             pvclock_copy_into_read_base(pvtk, &tk->tkr_mono,
> > +                 &pvtk->tkr_mono);
> > +             pvclock_copy_into_read_base(pvtk, &tk->tkr_raw, &pvtk->tkr_raw);
> > +
> > +             tk->xtime_sec = pvtk->xtime_sec;
> > +             tk->ktime_sec = pvtk->ktime_sec;
> > +             tk->wall_to_monotonic.tv_sec = pvtk->wall_to_monotonic_sec;
> > +             tk->wall_to_monotonic.tv_nsec = pvtk->wall_to_monotonic_nsec;
> > +             tk->offs_real = pvtk->offs_real;
> > +             tk->offs_boot = pvtk->offs_boot;
> > +             tk->offs_tai = pvtk->offs_tai;
> > +             tk->raw_sec = pvtk->raw_sec;
> > +     } while (pvtk_read_retry(pvtk, gen));
> > +}
> > +
>
> Should you write an "enabled value" (basically the flags) into pvtk as well?

I think we have that already (pvtk->flags).
I'll change the if statement above to use pvtk instead of pv_timekeeper.

> > +kvm_hostclock_init(void)
> > +{
> > +     unsigned long pa;
> > +
> > +     pa = __pa(&pv_timekeeper);
> > +     wrmsrl(MSR_KVM_TIMEKEEPER_EN, pa);
>
>
> As Vitaly said, a new CPUID bit must be defined in
> Documentation/virt/kvm/cpuid.txt, and used here.  Also please make bit 0
> an enable bit.

I think I might not be able to move the enable bit to 0 because we
need the generation count (pvclock_timekeeper.gen) to be the first
word of the struct due to the way we copy the data to userspace,
similarly to how kvm_setup_pvclock_page() does it.

I also noticed the comment in kvm_copy_into_pvtk() references the
wrong function, which I will fix in the next revision.

Thanks for the feedback.

-- Suleiman