linux-kernel - Re: [kvm-devel] 2.6.23.1-rt4 and kvm 48

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Date:	Tue, 30 Oct 2007 10:49:30 +0100
From:	Jan Kiszka <jan.kiszka@...mens.com>
To:	Carsten Emde <Carsten.Emde@...dl.org>
CC:	Avi Kivity <avi@...ranet.com>, kvm-devel@...ts.sourceforge.net,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [kvm-devel] 2.6.23.1-rt4 and kvm 48

Carsten Emde wrote:
> Avi Kivity wrote:
>> David Brown wrote:
>>> I thought I'd try out the realtime patch set and it didn't work at all
>>> with kvm. The console didn't dump anything and the system completely
>>> locked up.
> Up to now, the unmodified kvm module never worked with any RT kernel. 
> This would only change, if RT patched kernels were routinely used in the 
> kvm release tests which is, for the time being, not the case.

As far as I understood (but didn't re-test yet), latest kvm release does
include all features required to run over -rt. This was at least the
case for kvm-git I picked up last week and merged into 2.6.23-rt1. The
only issue is that out-of-tree kvm doesn't enable the preemption
notifiers (maybe there were one or two further minor issues, I think to
recall missing smp_ops exports being one, but these were not -rt related).

> 
> However, tglx once provided a hand-crafted kvm version that can be used 
> together with a selected RT kernel -> see "Latest Stable" 
> (http://www.osadl.org/Latest-Stable.latest-stable-realtime-kernel.0.html) 
> realtime kernel. It is based on kernel 2.6.21.6-rt21 and kvm-28.
> 
> We used these modules to carry out a number of "proof of concept" tests 
> which were very successful. The realtime capabilities of the host system 
> remained unchanged irrespective of whether the kvm guest system (no 
> graphics) was running or not. This was even the case while the guest 
> system was booting.

Interesting result - you've read about the wbinvd issues? Is there no
wbinvd in the bios shipped with older kvm? Which VM extension did you
test, both Intel and AMD? I would bet that your X issues are due to the
same effect. X startup/shutdown involves a lot of wbinvd calls on my
test boxes.

To sum up my findings: kvm over whatever RT kernel is risky
(latency-wise) unless you can audit or even para-virtualise your guest
OS /wrt wbinvd. Maybe we can discuss this also in Linz by the end of the
week. :)

Jan

-- 
Siemens AG, Corporate Technology, CT SE 2
Corporate Competence Center Embedded Linux
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