Re: assign-dev: Purpose of interrupt_work

[Gleb Natapov <gleb@redhat.com>]

On Mon, Oct 12, 2009 at 11:04:23AM +0200, Jan Kiszka wrote:
> Gleb Natapov wrote:
> > On Mon, Oct 12, 2009 at 10:16:56AM +0200, Jan Kiszka wrote:
> >>>>> Apic is lockless. For ioapic/pic I used spinlocks initially, but Avi
> >>>>> prefers mutexes. Theoretically it is possible to make them lockless,
> >>>>> but code will be complex and eventually more slow, since more then two
> >>>>> atomic operation will be used on irq injection path.
> >>>> Well, lockless is another thing.
> >>>>
> >>>> But also converting to spinlocks would indeed add some overhead:
> >>>> irqsave/restore. But I wonder if this isn't worth it, at least when
> >>>> looking at the (supposed to be fast) device passthrough scenario which
> >>>> would be simpler and faster.
> >>>>
> >>> Avi's point in favor of mutex is: they are as fast as spinlocks when
> >>> congested and allows preemption when held.
> >> ...but require scheduler activity in case of dev-passthrough, which is
> >> surely playing in a different league.
> >>
> > I'd rather remove dev-passthrough completely than continue adding hack upon hack
> > upon hack to make is some times kinda sorta work :)
> 
> Hmm, is this code not needed for the VT-d & Co. case? Or what is the
> alternative?
> 
The smile there is for a reason. My dislike of dev-passthrough will not
make it go away. Alternatives? I don't know what HW makers plans are, but
with interrupt remapping + APIC vitalization in HW theoretically it is
possible to inject interrupts directly from device to virtual machine.

--
			Gleb.