Re: [Qemu-trivial] [Qemu-ppc] [Qemu-devel] [PATCH] RFC kvm irqfd: add directly mapped MSI IRQ support

[Alex Williamson]

On Sun, 2013-06-23 at 16:58 -0500, Anthony Liguori wrote:
> On Sun, Jun 23, 2013 at 4:39 PM, Benjamin Herrenschmidt
> <address@hidden> wrote:
> > On Sun, 2013-06-23 at 10:02 -0500, Anthony Liguori wrote:
> >> It doesn't make a lot of sense to have Power do MSI decoding in
> >> userspace while x86 does it in the kernel.  I assume the kernel
> >> doesn't have enough information to do the translation on Power?
> >
> > Well, it would need the host bridge which it doesn't have and it
> > would need a concept of mapping that to interrupt numbers which
> > we don't want in the kernel.
> >
> > We can keep the kernel side a LOT simpler (completely avoiding
> > the whole route bloatware) by sticking to our "simple" direct map kernel
> > stuff and doing the mapping from address/data to interrupts in qemu, so
> > we'll stick to that.
> 
> Yeah, but none of this is Power specific...  so we can do the same for x86, 
> no?
> 
> I'm still trying to wrap my head around why we need MSI knowledge at
> all in the kernel for x86.  I presume it's to fast-path irqfd when
> doing vhost?

Or device assignment.  Any paths where we want to inject an MSI
interrupt without going through userspace.  Thanks,

Alex