Re: KVM and Prempt?

[Jan Kiszka <jan.kiszka@siemens.com>]

Sven-Thorsten Dietrich wrote:
> On Mon, 2007-10-22 at 09:01 +0200, Back, Michael (ext) wrote:
>> Hallo,
>> I tried to run Windows XP with KVM on Linux 2.6.31.1 on a 
> 
> You mean .21.1 ? 

Classic typo I interestingly also did several times the last week. :)

> 
>> AMD Opteron and on a Intel Xeon, on both it works fine!
> 
>> After this test I patch the kernel with the current prempt-patch and on
>> both it doesn't works! 
> 
> Did you try against 2.6.23-rt1.

kvm in -rt1 is not usable. It's too old, lacking PREEMPT_NOTIFIER
support, thus quickly triggering lockdep.

> 
> If you must stay on .21, you might have some other issues with the AMD
> and NUMA.
> 
> At the very least, you will need to apply the attached patch from git
> somehow, although this patch is against a new scheduler post 2.6.22, so
> good luck :)

<--snip-->

Those patches are already mainline... :->

What you rather need are latest kvm patches, or - if building the kvm
distribution out of tree - a patch to enabled CONFIG_PREEMPT_NOTIFIERS
unconditionally:

--- linux-2.6.23.1-rt/kernel/Kconfig.preempt.orig
+++ linux-2.6.23.1-rt/kernel/Kconfig.preempt
@@ -136,6 +136,7 @@

 config PREEMPT_NOTIFIERS
 	bool
+	default y

 config PREEMPT_BKL
 	bool


Still, I'm seeing oopses here (more precisely, lock validator
complaints), but I need to re-test, better using kvm from git instead of
kvm-48.

Beyond this, I'm struggling to understand 300-400 us vm-exit latencies
(over Intel VMX), which appear to be independent of the underlying
system. See kvm-devel. Such latencies would limit the RT usability of
kvm - unless you spent dedicated CPUs.

Jan

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