RE: [Xen-devel] [PATCH] [RFC] Building guests on monotonic Xen system time

["Dan Magenheimer" <dan.magenheimer@xxxxxxxxxx>]

Thanks for the review and the modifications!

It does allow time to temporarily deviate across VCPUs, but
doesn't allow them to "apparently" deviate; that is a VCPU
can never observe or set a time which is older than the time
that another VCPU previously observed or set.

Unless you can guarantee this, there's no way to avoid "Time
going backwards" errors, is there?

Admittedly, I don't fully understand the inner workings of
all the timer modes, but if this doesn't fly for one or more
of the modes, doesn't this mean it is impossible for those
timer modes to guarantee monotonicity -- at least without
also requiring gang scheduling?

Thanks,
Dan

P.S. I had left the naming as hvm_get_guest_time_mono()
because I wasn't sure there would never be a version
needed that was non-mono, so thought it would be better
to be explicit in the name.  (I was especially sensitized
to this after having just completed teasing apart the
hvm_get_guest_tsc() from hvm_get_guest_time() ;-)

> -----Original Message-----
> From: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx
> [mailto:xen-devel-bounces@xxxxxxxxxxxxxxxxxxx]On Behalf Of Keir Fraser
> Sent: Thursday, May 22, 2008 2:46 AM
> To: dan.magenheimer@xxxxxxxxxx; Xen-Devel (E-mail)
> Subject: Re: [Xen-devel] [PATCH] [RFC] Building guests on 
> monotonic Xen
> system time
> 
> 
> Attached is a modified version (really just renames). However the new
> hvm_set_guest_time() interface doesn't work for vpt.c timer 
> modes which
> allow progress of time to temporarily deviate across VCPUs. I 
> guess you
> don't use those modes so might not notice this issue. I think 
> hvm guest time
> handling may need to be integrated into vpt.c so that correct 
> handling can
> be applied for the various different types of timer mode. 
> Having a blanket
> per-domain stime offset and enforcing monotonicity regardless 
> of timer mode
> doesn't fly.
> 
>  -- Keir
> 
> On 21/5/08 20:01, "Dan Magenheimer" 
> <dan.magenheimer@xxxxxxxxxx> wrote:
> 
> > OK, I think this version is ready for prime-time.
> >
> > Keir, please check-in to unstable.
> >
> > [PATCH] Build hvm guest platform timers on monotonic Xen system time
> >
> > This patch moves hvm platform timers from underlying 
> physical CPU TSC
> > to Xen system time and ensures monotonicity.  TSC on many 
> systems may
> > skew between processors, thus hvm platform timer reads were not
> > monotonic which led to "Time going backwards" problems.
> >
> > Signed-off-by: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>
> >
> >> -----Original Message-----
> >> From: Keir Fraser [mailto:keir.fraser@xxxxxxxxxxxxx]
> >> Sent: Tuesday, May 20, 2008 1:37 AM
> >> To: dan.magenheimer@xxxxxxxxxx; Xen-Devel (E-mail)
> >> Subject: Re: [Xen-devel] [PATCH] [RFC] Building guests on
> >> monotonic Xen
> >> system time
> >>
> >>
> >> On 19/5/08 19:27, "Dan Magenheimer"
> >> <dan.magenheimer@xxxxxxxxxx> wrote:
> >>
> >>> Why? Scaling and adjusting of xen-time-based-tsc will
> >>> be very difficult to coordinate with processor-based-tsc.
> >>> We need to always ensure that A < B < C for a guest
> >>> executing:
> >>>
> >>> rdtsc(A) /* untrapped */
> >>> emulated_rdtsc(B)
> >>> rdtsc(C) /* untrapped */
> >>>
> >>> Further, OS's use TSC as a highest-resolution time source
> >>> with knowledge that TSCs on different processors may
> >>> not be synchronized, whereas they assume that a platform
> >>> timer is one-per-system and monotonically increasing.
> >>>
> >>> Keir, if you disagree and see guest-TSC-on-Xen-system-time
> >>> as an absolute must, please let me know.
> >>
> >> I am inclined to say we should have a
> >> guest-TSC-on-system-time mode where
> >> *all* RDTSC executions get trapped. This would at least be 
> useful as a
> >> baseline for tracking down guest time problems, and also provide a
> >> guaranteed stable TSC timesource for those who care about
> >> that more than
> >> pure performance.
> >>
> >> *However* I would agree that, with TSC virtualisation as it
> >> currently is,
> >> there actually isn't really a way to build guest TSC on Xen
> >> system time
> >> without periodically warping the TSC back or forth. The guest
> >> TSC runs at
> >> the host TSC rate and that is that!
> >>
> >> I think my original point was that at least we should not
> >> build all our
> >> other virtual time sources on this dodgy 'guest TSC'. :-)
> >>
> >>  -- Keir
> >>
> >>
> >>
> 
> 
>


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