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Re: [Xen-devel] Re: [patch 14/21] Xen-paravirt: Add XEN config options and disableunsupported config options.

On 02/16/2007 01:51 PM, Zachary Amsden wrote:
Keir Fraser wrote:
On 16/2/07 17:46, "Jeremy Fitzhardinge" <jeremy@xxxxxxxx> wrote:

Keir Fraser wrote:
This initial patchset does not include save/restore support anyway, so in
fact it would be consistent to have CONFIG_PREEMPT configurable. I'm sure
that we are going to have some nasty bugs to fix up as a result, but we
can't fix them until we find them! Then we can convert our save/restore code
to use the freezer before submitting it for inclusion.
OK.  So that makes the only config restriction the 100Hz ticks.
We can extend the Xen timer interface quite easily and get rid of this one
too. In fact it doesn't *much* matter if the CONFIG_HZ differs from the Xen
ticker rate -- we modified the Linux timer ISR to handle timer interrupts at
arbitrary times already. The only drawback is that jiffies updates in burts
if CONFIG_HZ is higher than the actual tick rate, and this can affect some
calibration constants and cause Linux to print out some weird values at

That's why we'd very much like to get a get_cpu_speed paravirt-op implemented. I think this would be useful to work around these problems for Xen as well.

Yes, and regardless of whether you run your periodic timer slower than HZ, calibrating time in a VM is always difficult due to the fact the kernel is time sharing the physical cpu. Why not just ask the underlying hypervisor?

Really, Time is one of those areas where paravirtualization makes a lot of sense, no matter how much virtualization hardware you have crammed into your cpu....

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