Re: [PATCH] paravirt.h

[Zachary Amsden <zach@vmware.com>]

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