Re: [PATCH] paravirt.h

Zachary Amsden
2006-08-23T08:38:41Z

Andi Kleen wrote:
>
>
>> 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).
>>
>
> stop machine deals with preemption. If it didn't it would be unusable
> for the purposes the kernel uses it right now (cpu hotplug, module unloading etc.)
>

Yes, but it can't move pre-empted threads out of a particularly
dangerous EIP (like a piece of code we're about to patch over). Or
perhaps I am misunderstanding how it deals with preemption, and what it
really does is make sure all threads are in userspace or sleep state...
which in that case is perfectly fine.

> and machine checks. debug traps -- i assume you mean kernel debuggers --
> sounds like something that cannot be really controlled though.
>
> How do you control a debugger from the debugee?
>
> I don't think NMI/MCEs are a problem though because NMIs (at least oprofile/nmi watchdog)
> and MCEs all just have global state that can be changed on a single CPU.
>

But with paravirt-ops, that global state may include local CPU state, in
which paravirt-ops is intimately involved. So they could interrupt in
the middle of the patching code, then attempt a paravirt_ops call, which
is in an undefined state until the patching is complete. And I would
highly expect the debugger to mess with debug registers, which is a
paravirt op. NMIs can do plenty of dangerous things to local state as
well - reading and writing MSRs or performance counters I would imagine
to be quite useful.

Zach