On Tue, 2025-01-07 at 11:07 -0500, Michael S. Tsirkin wrote: > On Thu, Dec 19, 2024 at 05:24:11PM +0100, David Woodhouse wrote: > > From: David Woodhouse <d...@amazon.co.uk> > > > > The system GSIs are not designed for sharing. One device might assert a > > shared interrupt with qemu_set_irq() and another might deassert it, and > > the level from the first device is lost. > > > > This could be solved by using a multiplexer which functions as an OR > > gate, much like the PCI code already implements for pci_set_irq() for > > muxing the INTx lines. > > > > Alternatively, it could be solved by having a 'resample' callback which > > is invoked when the interrupt is acked at the interrupt controller, and > > causes the devices to re-trigger the interrupt if it should still be > > pending. This is the model that VFIO in Linux uses, with a 'resampler' > > eventfd that actually unmasks the interrupt on the hardware device and > > thus triggers a new interrupt from it if needed. QEMU currently doesn't > > use that VFIO interface correctly, and just bashes on the resampler for > > every MMIO access to the device "just in case". > > > > This does neither of those. The Xen event channel GSI support *already* > > has hooks into the PC gsi_handler() code, for routing GSIs to PIRQs. So > > we can implement the logical OR of the external input (from PCI INTx, > > serial etc.) with the Xen event channel GSI by allowing that existing > > hook to modify the 'level' being asserted. > > > > Closes: https://gitlab.com/qemu-project/qemu/-/issues/2731 > > Reported-by: Thomas Huth <th...@redhat.com> > > Signed-off-by: David Woodhouse <d...@amazon.co.uk> > > Xen things so feel free to merge. > > Acked-by: Michael S. Tsirkin <m...@redhat.com>
Thanks.
Further testing shows I need one minor fix...
> > @@ -1596,7 +1607,7 @@ static int allocate_pirq(XenEvtchnState *s, int type,
> > int gsi)
> > return pirq;
> > }
> >
> > -bool xen_evtchn_set_gsi(int gsi, int level)
> > +bool xen_evtchn_set_gsi(int gsi, int *level)
> > {
> > XenEvtchnState *s = xen_evtchn_singleton;
> > int pirq;
...
@@ -1628,7 +1656,7 @@ bool xen_evtchn_set_gsi(int gsi, int level)
return false;
}
- if (level) {
+ if (*level) {
int port = s->pirq[pirq].port;
s->pirq_gsi_set |= (1U << gsi);
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