Rusty Russell wrote:

On Tue, 2006-08-22 at 15:02 -0700, Zachary Amsden wrote:
  

quoted

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