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

[Michael S. Tsirkin]

On Wed, May 20, 2009 at 11:26:42PM +0300, Blue Swirl wrote:
> On 5/20/09, Michael S. Tsirkin <address@hidden> wrote:
> > 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.
> 
> No, but I think the Sparc situation is the same, the packet data is
> static for the interrupt source in question.

So, ok, we could add data update callback and then MSI and sparc
would do their thing there. I'm not convinced I like all this
play with untyped buffers, do you think it's helpful?

If yes, maybe I'll try to code it up and see how does it look.

-- 
MST