Re: [Qemu-devel] [PATCH v3 12/13] q35: ioapic: add support for split irqchip and irqfd

[Peter Xu <peterx@redhat.com>]

On Sun, Apr 17, 2016 at 12:45:03PM +0300, Michael S. Tsirkin wrote:
> On Sat, Apr 16, 2016 at 07:44:12PM -0700, Jan Kiszka wrote:
> > On 2016-04-14 20:31, Peter Xu wrote:
[...]
> > > diff --git a/hw/intc/ioapic.c b/hw/intc/ioapic.c
> > > index 84e8948..b993bd0 100644
> > > --- a/hw/intc/ioapic.c
> > > +++ b/hw/intc/ioapic.c
> > > @@ -182,34 +182,23 @@ static void ioapic_update_kvm_routes(IOAPICCommonState *s)
> > >  {
> > >  #ifdef CONFIG_KVM
> > >      int i;
> > > +    int ret;
> > >  
> > >      if (kvm_irqchip_is_split()) {
> > >          for (i = 0; i < IOAPIC_NUM_PINS; i++) {
> > > -            uint64_t entry = s->ioredtbl[i];
> > > -            uint8_t trig_mode;
> > > -            uint8_t delivery_mode;
> > > -            uint8_t dest;
> > > -            uint8_t dest_mode;
> > > -            uint64_t pin_polarity;
> > > -            MSIMessage msg;
> > > -
> > > -            trig_mode = ((entry >> IOAPIC_LVT_TRIGGER_MODE_SHIFT) & 1);
> > > -            dest = entry >> IOAPIC_LVT_DEST_SHIFT;
> > > -            dest_mode = (entry >> IOAPIC_LVT_DEST_MODE_SHIFT) & 1;
> > > -            pin_polarity = (entry >> IOAPIC_LVT_TRIGGER_MODE_SHIFT) & 1;
> > > -            delivery_mode =
> > > -                (entry >> IOAPIC_LVT_DELIV_MODE_SHIFT) & IOAPIC_DM_MASK;
> > > -
> > > -            msg.address = APIC_DEFAULT_ADDRESS;
> > > -            msg.address |= dest_mode << 2;
> > > -            msg.address |= dest << 12;
> > > -
> > > -            msg.data = entry & IOAPIC_VECTOR_MASK;
> > > -            msg.data |= delivery_mode << APIC_DELIVERY_MODE_SHIFT;
> > > -            msg.data |= pin_polarity << APIC_POLARITY_SHIFT;
> > > -            msg.data |= trig_mode << APIC_TRIG_MODE_SHIFT;
> > > -
> > > -            kvm_irqchip_update_msi_route(kvm_state, i, msg, NULL);
> > > +            MSIMessage src, dst;
> > > +            struct ioapic_entry_info info;
> > > +            ioapic_entry_parse(s->ioredtbl[i], &info);
> > > +            src.address = info.addr;
> > > +            src.data = info.data;
> > > +            /* We update kernel irqchip routes with translated
> > > +             * results. */
> > > +            ret = s->int_remap(s->iommu, &src, &dst);
> > > +            if (ret) {
> > > +                DPRINTF("Int remap failed: %d, drop interrupt\n", ret);
> > > +                continue;
> > > +            }
> > > +            kvm_irqchip_update_msi_route(kvm_state, i, dst, NULL);
> > 
> > The need to hook here makes me wonder if we can't inject IOAPIC
> > interrupts via KVM_SIGNAL_MSI (abstracted by kvm_irqchip_send_msi, but
> > that will pick the fast-path on kernels supporting split irqchip)
> > instead of open-coding the route changes. If we translated the IOAPIC
> > outputs always into MSIs, the need for special-casing split irqchip
> > would be gone, and the need for hooking here for IR just as well.

Hi, Jan,

IIUC, this can be achieved by removing lines in ioapic_service():

-#ifdef CONFIG_KVM
-                if (kvm_irqchip_is_split()) {
-                    if (trig_mode == IOAPIC_TRIGGER_EDGE) {
-                        kvm_set_irq(kvm_state, i, 1);
-                        kvm_set_irq(kvm_state, i, 0);
-                    } else {
-                        if (!coalesce) {
-                            kvm_set_irq(kvm_state, i, 1);
-                        }
-                    }
-                    continue;
-                }
-#else
-                (void)coalesce;
-#endif

So that QEMU will automatically select the correct way to notify the
interrupt depending on whether "apic" or "kvm-apic" is used.

I suppose this is a good way to do if we are to support split
irqchip only. However, what if we move on to support irqfd and
vhost? If so, we may still need to update kernel entries into
translated ones, right? Or... did I miss anything?

> 
> Will work for irqfd as well.

Michael,

Could you help explain how could I support irqfd as well using Jan's
method mentioned above?

Thanks in advance (to both :).

-- peterx