Re: [PATCH] paravirt.h

Zachary Amsden
2006-08-22T22:02:55Z

Andi Kleen wrote:
> Zachary Amsden <zach@vmware.com> writes:
>
>> That is a really nasty problem. You need a synchronization primitive
>> which guarantees a flat stack, so you can't do it in the interrupt
>> handler as I have tried to do. I'll bang my head on it awhile. In
>> the meantime, were there ever any solutions to the syscall patching
>> problem that might lend me a clue as to what to do (or not to do, or
>> impossible?).
>>
>
> stop_machine_run() solves the problem I think. It is currently not
> exported though. I don't think there's anything in there that couldn't
> be reimplemented in a module, but then we could also just export it
> if there's a useful user.
>

Well, I don't think anything is sufficient for a preemptible kernel. I
think that's just plain not going to work. You could have a kernel
thread that got preempted in a paravirt-op patch point, and making all
the patch points non-preempt is probably a non-starter (either +12 bytes
each or no native inlining). Finding out after the fact that you have a
kernel thread that was preempted in a patch point is very hard work, but
it is possible. The fixing it up is where you need to take liberties
with reality.

stop_machine_run() is almost what I want, but even that is not
sufficient. You also need to disable NMIs and debug traps, which is
pretty hairy, but doable. The problem with stop_machine_run() is that I
don't just want the kernel to halt running on remote CPUs, I want the
kernel on all CPUs to actually do something simultaneously - the entry
into paravirt mode requires a hypervisor call on each CPU, and
stop_machine() doesn't provide a facility to fire a callback on each CPU
from the stopmachine state.

Since this code is so rather, um, custom, I was going to reimplement
stop_machine in the module.

Zach