Re: Possible APIC conflict with 2.6.38.8 Kernel

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

On 2012-02-07 16:25, Simon wrote:
> We're building a virtual windows 7 x64 model on top of a custom Linux OS.
> We're currently using 2.6.38.8 kernel.
> I was rather attracted to the new kvm-apic/qemu-kvm merge you've currently
> made available, as this may hold the answer to a stubborn problem; 
> every time we load or attempt to create a windows 7 image while 
> our RTAI(real time) base is running, at an early stage of loading 
> windows the real time breaks and stops dead.
> 
> We suspect this is due to APIC conflicts between RTAI and Qemu-kvm.
> 
> I've read with interest your internal debate on making kvm-apic options
> available, and I wonder if these now exist and if so, what they are
> (the online help may not have kept pace).
> Note that these are 64-bit OS's. With older kernels and 0.13.0 
> with 32 bit XP, we didn't have this problem.

That's Russian roulette. You can't simply use KVM (or any other
virtualization subsystem) on an I-pipe-patched host kernel. See [1] for
the requirements on I-pipe (will be contained a bit differently in
upcoming 3.x patches for x86) and [2] for the real-time extension on
top. We implemented it for Xenomai, but RTAI should be patchable as well
- in principle.

Note that there is potentially a FPU-related guest corruption with these
patches remaining, but I wasn't able to reproduce it yet. May depend on
host CPU features.

Just to clarify: this is not really a topic of KVM or upstream Linux -
or even QEMU. The I-pipe patch changes the semantics of low-level
interrupt masking in Linux and opens new context switching points. Thus,
it is responsible for making the KVM subsystem aware of this and for
coordination with the real-time extension.

Jan

[1] http://thread.gmane.org/gmane.linux.kernel.adeos.general/1898
[2]
http://git.xenomai.org/?p=xenomai-2.6.git;a=commitdiff;h=35505fe10c0dae3d1eab8a409b978d338b84b89e

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