Re: [PATCH] paravirt.h

Zachary Amsden
2006-08-23T02:12:13Z

Rusty Russell wrote:
> On Tue, 2006-08-22 at 15:02 -0700, Zachary Amsden 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
>>
>
> Patching over the 6 native cases is actually not that bad: they're
> listed below (each one has trailing noops).
>
> cli
> sti
> push %eax; popf
> pushf; pop %eax
> pushf; pop %eax; cli
> iret
> sti; sysexit
>
> If you're at the first insn you don't have to do anything, since you're
> about to replace that code. If you're in the noops, you can just
> advance EIP to the end. You can't be preempted between sti and sysexit,
> since we only use that when interrupts are already disabled. And
> reversing either "push %eax" or "pushf; pop %eax" is fairly easy.
>
> Depending on your hypervisor, you might need to catch those threads who
> are currently doing the paravirt_ops function calls, as well. This
> introduces more (and more complex) cases.
>

Yes, but the problem gets far worse. You don't need to worry about just
those. You need to worry about all that C code that runs in the native
paravirt-ops as well, because you could have preempted it in the middle
of a callout. And the paravirt_ops code isn't isolated in a separate
section (though it well could be).

Zach