linux-kernel - Re: [PATCH 4/4] KVM: x86: track actual TSC frequency from the timekeeper struct

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Message-ID: <20160216134816.GA21944@amt.cnet>
Date:	Tue, 16 Feb 2016 14:48:17 +0100
From:	Marcelo Tosatti <mtosatti@...hat.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH 4/4] KVM: x86: track actual TSC frequency from the
 timekeeper struct

On Mon, Feb 08, 2016 at 04:18:31PM +0100, Paolo Bonzini wrote:
> When an NTP server is running, it may adjust the time substantially
> compared to the "official" frequency of the TSC.  A 12 ppm change
> sums up to one second per day.
> 
> This already shows up if the guest compares kvmclock with e.g. the
> PM timer.  It shows up even more once we add support for the Hyper-V
> TSC page, because the guest expects it to be in sync with the time
> reference counter; effectively the time reference counter is just a
> slow path to access the same clock that is in the TSC page.
> 
> Therefore, we want kvmclock to provide the host kernel's
> ktime_get_boot_ns() value, at least if the master clock is active.
> To do so, reverse-compute the host's "actual" TSC frequency from
> pvclock_gtod_data and return it from kvm_get_time_and_clockread.

Paolo,

You'd have to generate an update to the guest structures as well, 
to reflect the new {mult,shift} values calculated by the host. 
Here:

        /* disable master clock if host does not trust, or does not
         * use, TSC clocksource
         */
        if (gtod->clock.vclock_mode != VCLOCK_TSC &&
            atomic_read(&kvm_guest_has_master_clock) != 0)
                queue_work(system_long_wq, &pvclock_gtod_work);

No?

At first, i'm afraid this might be heavy, so it might be interesting
to rate limit the update operation.