Re: [Xen-devel] [PATCH] xen: always set the sched clock as unstable

[Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>]

> From: Tim Deegan [mailto:tim@xxxxxxx]
> Subject: Re: [Xen-devel] [PATCH] xen: always set the sched clock as unstable
> 
> At 09:05 -0700 on 16 Apr (1334567132), Dan Magenheimer wrote:
> > Hmmm... I spent a great deal of time on TSC support in the hypervisor
> > 2-3 years ago.  I worked primarily on PV, but Intel supposedly was tracking
> > everything on HVM as well.  There's most likely a bug or two still lurking
> > but, for all guests, with the default tsc_mode, TSC is provided by Xen
> > as an absolutely stable clock source.  If Xen determines that the underlying
> > hardware declares that TSC is stable, guest rdtsc instructions are not 
> > trapped.
> > If it is not, Xen emulates all guest rdtsc instructions.  After a migration 
> > or
> > save/restore, TSC is always emulated.  The result is (ignoring possible
> > bugs) that TSC as provided by Xen is a) monotonic; b) synchronized across
> > CPUs; and c) constant rate.  Even across migration/save/restore.
> 
> AIUI, this thread is about the PV-time clock source, not about the TSC
> itself.  Even if the TSC is emulated (or in some other way made
> "stable") the PV wallclock is not necessarily stable across migration.
> But since migration is controlled by the kernel, presumably the kernel
> can DTRT about it.

Under what circumstances is PV wallclock not stable across migration?

> > In fact, it might be wise for a Xen-savvy kernel to check to see
> > if it is running on Xen-4.0+ and, if so, force clocksource=tsc
> > and tsc=reliable.
> 
> That seems like overdoing it.  Certainly it's not OK unless it can also
> check that Xen is providing a stable TSC (i.e. that tscmode==1).

Xen guarantees a stable TSC for the default (tsc_mode==0) also.

If the vm.cfg file explicitly sets a guest tsc_mode==2, you are correct
that pvclock is still necessary.  But as the documentation says:
tsc_mode==2 should be set if "it is certain that all apps running in this
VM are TSC-resilient and highest performance is required".  In
the case we are talking about, the PV guest kernel itself isn't TSC-
resilient!

In any case, IIRC, there is a pvcpuid instruction to determine the
tsc_mode, so when the upstream kernel checks for Xen 4.0+, it could
also check to ensure the tsc_mode wasn't overridden and set to 2.
If it is set to 2, TSC should not be an available clocksource,
as the guest kernel would break on migration/save/restore.

> In the case where the PV clock has been selected, can it not be marked
> unstable without also marking the TSC unstable?

I'm not sure I understand...

Are you talking about the HVM case of an upstream kernel, maybe
when the clocksource is manually overridden on the kernel command
line or after boot with sysfs?

If pvclock is necessary (e.g. old Xen), how would it be
marked unstable? (I didn't know there was code to do that.)

Dan

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