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

[Michael S. Tsirkin]

On Wed, May 20, 2009 at 11:18:56PM +0300, Blue Swirl wrote:
> On 5/20/09, Michael S. Tsirkin <address@hidden> 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.
> 
> But maybe this API would be useful for both MSI and mondo packets:
> /* Get vector data stored in the irq */
> void *qemu_irq_get_data(qemu_irq irq);

Hmm. Yes, it would work for MSI.
However I am thinking ahead to kvm support, which needs to
update the irqchip (with a system call) on each state update.
So 
- I really need a callback on data update
- data update might fail

Which if added to the generic API would make it too complex,
IMO.

> /* Set vector data stored in the irq */
> void qemu_irq_set_data(qemu_irq irq, void *data);
> You'd use a struct msi_state as the opaque (not using the array
> indexed by irq number), I'd use some kind of
> struct mondo {
>  uint64_t packet[8];
> };
> 
> Instead of ioapic_update_msi(), you'd call qemu_irq_set_data().

I guess it's a matter of taste, personally I prefer type-safe functions
to hiding everything in typeless void* buffers.

-- 
MST