Re: [Qemu-devel] [PATCH] qemu: msi irq allocation api

[Michael S. Tsirkin]

On Wed, May 20, 2009 at 11:02:24PM +0300, Michael S. Tsirkin wrote:
> On Wed, May 20, 2009 at 09:38:58PM +0300, Blue Swirl wrote:
> > On 5/20/09, Michael S. Tsirkin <address@hidden> wrote:
> > > On Wed, May 20, 2009 at 08:44:31PM +0300, Blue Swirl wrote:
> > >  > On 5/20/09, Michael S. Tsirkin <address@hidden> wrote:
> > >  > > On Wed, May 20, 2009 at 08:21:01PM +0300, Blue Swirl wrote:
> > >  > >  > On 5/20/09, Michael S. Tsirkin <address@hidden> wrote:
> > >  > >  > > define api for allocating/setting up msi-x irqs, and for 
> > > updating them
> > >  > >  > >  with msi-x vector information, supply implementation in 
> > > ioapic. Please
> > >  > >  > >  comment on this API: I intend to port my msi-x patch to work 
> > > on top of
> > >  > >  > >  it.
> > >  > >  > >
> > >  > >  > >  Signed-off-by: Michael S. Tsirkin <address@hidden>
> > >  > >  >
> > >  > >  > Sparc64 also uses packets ("mondos", not implemented yet) for
> > >  > >  > interrupt vector data, there the packet size is 8 * 64 bits.
> > >  > >  > I think we should aim for a more generic API that covers this 
> > > case also.
> > >  > >
> > >  > >
> > >  > > Are you sure this is a good idea? MSI is tied to PCI, and PCI only 
> > > has
> > >  > >  MSI, not "mondos". What code would benefit from this abstraction?
> > >  >
> > >  > Sparc64 emulation, of course. I think also the API would be neater.
> > >
> > >
> > > Since "mondos" are not interrupts, why use irqs for them?
> > 
> > I just said above that they are used for interrupt vector data. What
> > makes you think they are not interrupts?
> 
> I'm sorry, I don't really know anything about sparc.
> All I am saying is that in PCI, interrupts never pass data,
> so qemu_set_irq as it is now, is a good API to send them.
> 
> For the sparc feature you describe, you probably want to add
> a message data parameter to qemu_set_irq, but it's not
> really useful for MSI.

Just to clarify, the main difference is that with MSI/MSI-X
both data and address fields are mostly static, modifying them
involves ioapic and device updates which might be an expensive
operation (e.g. with kvm, needs an extra system call).

So I don't think it makes sense to pass MSI-X data field
with each call to qemu_set_irq.

-- 
MST