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
