Re: [PATCH] [RFC] Building guests on monotonic Xen system time

[Keir Fraser <keir.fraser@eu.citrix.com>]

On 23/5/08 23:44, "Dan Magenheimer" <dan.magenheimer@oracle.com> wrote:

> Here's another option:  I think delay_for_missed_ticks is
> the only mode where time must be set backwards.  All of the
> other modes track wallclock.  It looks possible that we
> could enforce monotonicity for the other three modes and
> not enforce it for delay_for_missed_ticks (which is,
> unfortunately, the default).
> 
> If this is acceptable to you (and you don't disagree with
> my "only mode...set backwards" conjecture), I'll try to
> code a patch for this.

I agree with your analysis. I couldn't remember if any modes other than 0
would require non-monotonic time across VCPUs, but it does appear that this
property is exclusive to mode 0.

> P.S. I think the answer is no, but is there any way to specify
> gang-scheduling for a domain?  If so, we could say if your
> OS is non-resilient to preemption and your application requires
> time monotonicity, you must specify gang scheduling.

There isn't currently a way to request gang scheduling. I think that is a
second step anyway. It sounds like you could simply turn off your
monotonicity checks for timer mode 0 and I'll be happy.

I have a pretty clear vision of what I want now, so I will make the
necessary adjustments to the patch. Then we can go from there if you think
further modifications are required.

 -- Keir