On Fri, 2010-04-23 at 14:15 +0200, Jan Kiszka wrote:
> [ dropping xenomai-help before going into details ]
> 
> Philippe Gerum wrote:
> > On Fri, 2010-03-12 at 16:25 +0100, Jan Kiszka wrote:
> >> Hi,
> >>
> >> this is still in the state "study", but it is working fairly nicely so far:
> >>
> >> These two patches harden latest KVM for use over I-pipe kernels and make
> >> Xenomai aware of the lazy host state restoring that KVM uses for
> >> performance reasons. The latter basically means calling the sched-out
> >> notifier that KVM registers with the kernel when switching from a Linux
> >> task to some shadow. This is safe in all recent versions of KVM and
> >> still gives nice KVM performance (that of KVM before 2.6.32) without
> >> significant impact on the RT latency (Note: if you have an old VT-x CPU,
> >> guest-issued wbinvd will ruin RT as it is not intercepted by the 
> >> hardware!).
> >>
> >> To test it, you need to apply the kernel patch on top of current kvm.git
> >> master [1], obtain kvm-kmod.git [2], run configure on it (assuming your
> >> host kernel is a Xenomai one, otherwise use --kerneldir) and then "make
> >> sync-kmod LINUX=/path/to/kvm.git". After a final make && make install,
> >> you will have recent kvm modules that are I-pipe aware. The Xenomai
> >> patch simply appies to the 2.5 tree. This has been tested with
> >> ipipe-2.6.32-x86-2.6-01 + [3] and Xenomai-2.5 git.
> >>
> >> Feedback welcome, specifically if you think it's worth integrating both
> >> patches into upstream. The kernel bits would make sense over some
> >> 2.6.33-x86, but additional work will be required to account for the
> >> user-return notifiers introduced with that release (kvm-kmod currently
> >> wraps them away for older kernels).
> > 
> > No concern on the final goal, running a Xenomai-enabled kernel
> > rock-solid over KVM is a must.
> > 
> > The KVM code ironing from the 1st patch looks fine to me, no big deal to
> > maintain AFAICS. I would be only concerned by the 2nd patch,
> > specifically how the KVM callout is invoked from the Xenomai context
> > switching code:
> > 
> > - depending on CONFIG_PREEMPT_NOTIFIERS is much broader than required; I
> > guess that CONFIG_KVM would be enough.
> 
> So far, only CONFIG_KVM enables CONFIG_PREEMPT_NOTIFIERS. Granted, this
> could change in the future. But letting our invocation depend on
> CONFIG_KVM would not automatically remove the need to review those new
> notifiers (BTW, there would be a fairly high probability that those will
> be of some use for Xenomai as well).
> 
> > 
> > - calling the KVM callout directly instead of going through the notifier
> > list would be more acceptable, so that we don't assume anything from the
> > non-KVM hooks (whether they exist or not), albeit we may assume that we
> > have complete information about which KVM callout has to be run for a
> > particular kernel version.
> 
> Possible, but hacky. We would have to
> 
> - export the callback from the KVM module
>   (this will also mean the nucleus will depend on CONFIG_KVM if the
>   latter is on)

Which is already the case for a number of knobs anyway (particularly on
x86*).

> - somehow get hold of the notifier entry (I have no clue how as they are
>   per-vcpu)
> - invoke the callback directly, passing that notifier entry
> 

This is what I had in mind in my post.

> or
> 
> - identify the KVM callback in the notifier chain and only call that one
>   when walking the list

I don't see any upside to this yet. If this is about context preparation
that would be done by the notification system, then we'd better off
mimicking it, instead of introducing kludges to reuse it.

> 
> The latter could be achieved by somehow tagging KVM notifiers in order
> to find them when walking the chain. Still quite some patching, and I'm
> not yet sure it's worth the safety gain.

The point is that we shall check whether our coupling to the KVM system
is correct, for each kernel version we want to support anyway. This
means that some preparation work has to be done, whether it is by
inspecting the possibly NMI-unsafe notifier hooks or the interface rules
to the KVM hook is not the most important thing here.

If you definition of "hacky" here means "ad hoc", in any case, any
implementation you could find would be hacky, because Xenomai introduces
a context switching spot in a kernel that does not expect it, and as
such, we do bypass the normal paths for this. Therefore, I see no way to
do this without exactly knowing the kernel/KVM context, on a per-release
basis.

> 
> Jan
> 


-- 
Philippe.



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