linux-kernel - Re: [PATCH 3/4] KVM: x86: introduce get_kvmclock

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Date:   Fri, 2 Sep 2016 18:37:59 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Roman Kagan <rkagan@...tuozzo.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, peterhornyack@...gle.com, rkrcmar@...hat.com,
        den@...nvz.org
Subject: Re: [PATCH 3/4] KVM: x86: introduce get_kvmclock_ns

On 02/09/2016 16:51, Roman Kagan wrote:
> On Fri, Sep 02, 2016 at 04:09:42PM +0200, Paolo Bonzini wrote:
>> On 02/09/2016 15:52, Roman Kagan wrote:
>> vs. using a single offset as in the TSC ref page is one nanosecond---and
>> the ref page only has a resolution of 100 ns.
> 
> AFAICS it's not a matter of resolution.  If one calculation flips from
> value T to T+1 at tsc1, while the other at tsc2, during the window
> between tsc1 and tsc2 we can have monotonicity violation.

Ok, tried "empirically" (throw numbers in a spreadsheet :)) and indeed
the maximum error is not 1 ns but 100 ns (1 unit in the time reference
count MSR).

You can get a flip between T/T+1 because the time reference counter may
be more precise with its rounding due to the separation between
tsc_timestamp and system_time.  This separation provides some extra
decimal digits to the offset, which the TSC page lacks.  For example:

51256391	tsc_timestamp
3311474323	tsc_to_system_mul
254246		system_time
-1		shift
-195054.1816	offset (computed exactly)

So the flip happens when the nanoseconds are around 81/82:

	tsc	   kvmclock	refcount  TSC page
	51256391   254246	2542	  2542
	51256483   254281	2542	  2542
	51256484   254281	2542	  2543
	51256486   254282	2542	  2543
	51256746   254382	2543	  2544

I'll change patch 4 to store the parameters and use them when accessing
the time reference counter MSR.  I'll still keep the procedure that goes
through kvmclock.  It's a bit more involved for the scale, but
vcpu->last_guest_tsc only provides a part of the offset computation; the
other half is vcpu->hv_clock.system_time and it's not stored anywhere.

Paolo