RE: [Xen-ia64-devel] SMP guest and itc

["Magenheimer, Dan (HP Labs Fort Collins)" <dan.magenheimer@xxxxxx>]

Just to briefly reiterate my opinion on the general
timer tick issue:  The vast majority of timer ticks
get immediately reflected to the currently executing
domain.   Since ticks are very frequent (though recently
reduced in Linux/ia64 from 1024/sec to 250/sec), there
needs to be a "fast path" that can easily determine
if this is a "immediately reflect" tick or not.  It
shouldn't be necessary to go through all the save/restore
overhead to call C and then march down linked
lists only to determine that this is indeed one of the
99% of ticks that should be immediately reflected.

That said, I agree that Xen needs to pre-synchronize ITC
for host SMP to work properly.  I don't think guest SMP
for Linux will force ITC to be paravirtualized, but
I guess Tristan will let us know if he discovers otherwise.

Dan

> -----Original Message-----
> From: xen-ia64-devel-bounces@xxxxxxxxxxxxxxxxxxx 
> [mailto:xen-ia64-devel-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf 
> Of Dong, Eddie
> Sent: Thursday, February 09, 2006 10:04 PM
> To: Williamson, Alex (Linux Kernel Dev); Tristan Gingold
> Cc: xen-ia64-devel@xxxxxxxxxxxxxxxxxxx
> Subject: RE: [Xen-ia64-devel] SMP guest and itc
> 
> Tristan:
>       This is a known issue and I ever disucssed time virtualization
> with Dan for a while. 
>       I prefer #1. That let domU see its ITC start from 0 at boot
> time? And further more, we need to support Windows eventually.
>       In the meantime, I think both #1 and #3 should co-exist. Yes,
> isn't the host ITC pre-synchronized in Xen?
> Eddie.
> 
> Alex Williamson wrote:
> > On Thu, 2006-02-09 at 08:32 +0100, Tristan Gingold wrote:
> >> Hi,
> >> 
> >> during boot, linux kernel tries to synchronize itc of all
> >> processors.  However currently itc is not virtualized:
> >> (XEN) vcpu_set_itc: Setting ar.itc is currently disabled
> >> 
> >> What should we do ?
> >> a) virtualize ITC
> >> b) para-virtualize ITC (ie, modifying linux kernel)
> >> c) pre-synchronize ITC
> >> 
> >> I really prefer c).
> > 
> >    I'm not sure I understand c).  Aren't the ITCs already
> > pre-synchronized by xen in smp_callin()?   Thanks,
> > 
> >     Alex
> 
> 
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