RE: [Xen-users] Problems with GPLPV network latency

["Robert Dunkley" <Robert@xxxxxxxxx>]

An email from a Keir Frasier I was sent a year or two ago:

The default timer_mode is that timer interrupts are queued up while an
HVM CPU is preempted (not running because some other VCPU is running).
These then get injected into the HVM CPU when it starts running again,
so that OSes that track time by 'counting ticks' keep track of wall
time. This default mode also delays all timers from the point of view of
that HVM guest so that apparent time (e.g., by reading HPET or ACPI PM
counters) does not make it clear that timer ticks are very late.

This holding back of timers confuses some OSes (actually it probably
hinders more than it helps -- it's just some versions of x86/64 Linux
which whine if they think ticks are getting delivered late). In
particular Windows would like all its ticks delivered, but it doesn't
mind getting them late -- and this is precisiely what timer_mode=1
implements.

For those who want to know what all timer modes do, see the HVMPTM_*
definitions and block comment in xen/include/public/hvm/params.h (may be
installed as /usr/include/xen/hvm/params.h).

Feel free to distribute this on xen-users if others have the same
question.


-----Original Message-----
From: xen-users-bounces@xxxxxxxxxxxxxxxxxxx
[mailto:xen-users-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of James Harper
Sent: 09 June 2010 14:03
To: Nick Couchman; xen-users@xxxxxxxxxxxxxxxxxxx; itobin@xxxxxxxxxxxxx
Subject: RE: [Xen-users] Problems with GPLPV network latency

> James,
> What config options adjust the timer in Xen?
> 

It's timer_mode I think. I'm not sure what all the different values mean
though.

James

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