Re: [PATCH v3] KVM: x86/xen: improve accuracy of Xen timers

[Sean Christopherson]

On Thu, Dec 14, 2023, David Woodhouse wrote:
> From: David Woodhouse <dwmw@xxxxxxxxxxxx>
> 
> A test program such as http://david.woodhou.se/timerlat.c confirms user
> reports that timers are increasingly inaccurate as the lifetime of a
> guest increases. Reporting the actual delay observed when asking for
> 100µs of sleep, it starts off OK on a newly-launched guest but gets
> worse over time, giving incorrect sleep times:
> 
> root@ip-10-0-193-21:~# ./timerlat -c -n 5
> 00000000 latency 103243/100000 (3.2430%)
> 00000001 latency 103243/100000 (3.2430%)
> 00000002 latency 103242/100000 (3.2420%)
> 00000003 latency 103245/100000 (3.2450%)
> 00000004 latency 103245/100000 (3.2450%)
> 
> The biggest problem is that get_kvmclock_ns() returns inaccurate values
> when the guest TSC is scaled. The guest sees a TSC value scaled from the
> host TSC by a mul/shift conversion (hopefully done in hardware). The
> guest then converts that guest TSC value into nanoseconds using the
> mul/shift conversion given to it by the KVM pvclock information.
> 
> But get_kvmclock_ns() performs only a single conversion directly from
> host TSC to nanoseconds, giving a different result. A test program at
> http://david.woodhou.se/tsdrift.c demonstrates the cumulative error
> over a day.
> 
> It's non-trivial to fix get_kvmclock_ns(), although I'll come back to
> that. The actual guest hv_clock is per-CPU, and *theoretically* each
> vCPU could be running at a *different* frequency. But this patch is
> needed anyway because...
> 
> The other issue with Xen timers was that the code would snapshot the
> host CLOCK_MONOTONIC at some point in time, and then... after a few
> interrupts may have occurred, some preemption perhaps... would also read
> the guest's kvmclock. Then it would proceed under the false assumption
> that those two happened at the *same* time. Any time which *actually*
> elapsed between reading the two clocks was introduced as inaccuracies
> in the time at which the timer fired.
> 
> Fix it to use a variant of kvm_get_time_and_clockread(), which reads the
> host TSC just *once*, then use the returned TSC value to calculate the
> kvmclock (making sure to do that the way the guest would instead of
> making the same mistake get_kvmclock_ns() does).
> 
> Sadly, hrtimers based on CLOCK_MONOTONIC_RAW are not supported, so Xen
> timers still have to use CLOCK_MONOTONIC. In practice the difference
> between the two won't matter over the timescales involved, as the
> *absolute* values don't matter; just the delta.
> 
> This does mean a new variant of kvm_get_time_and_clockread() is needed;
> called kvm_get_monotonic_and_clockread() because that's what it does.
> 
> Fixes: 536395260582 ("KVM: x86/xen: handle PV timers oneshot mode")
> Signed-off-by: David Woodhouse <dwmw@xxxxxxxxxxxx>
> ---

Dagnabbit, this one is corrupt too :-/