Re: [PATCH for-5.0] xive/kvm: Trigger interrupts from userspace

[Greg Kurz <groug@kaod.org>]

On Mon, 18 Nov 2019 16:37:16 +0100
Cédric Le Goater <clg@kaod.org> wrote:

> On 18/11/2019 16:12, Greg Kurz wrote:
> > When using the XIVE KVM device, the trigger page is directly accessible
> > in QEMU. Unlike with XICS, no need to ask KVM to fire the interrupt. A
> > simple store on the trigger page does the job.
> > 
> > Just call xive_esb_trigger().
> 
> Yes but the KVM XIVE device does a few other checks. 
> 
> It checks that the interrupt was correctly initialized at the KVM device
> level. We should be fine in QEMU which has similar checks.
> 

Yeah, not sure we can do much more than to trust the QEMU irq code to
pass us a valid srcno.

> It caches the LSI assertion level. We should be fine also because it is
> useless in KVM when using the XIVE native exploitation mode.
> 

Yeah, I see it is set in kvmppc_xive_native_set_source() (why?) but never
used anywhere else in book3s_xive_native.c.

> It checks it is not a passthru interrupt. Any idea on how to check this 
> condition under QEMU ? 
> 

AFAICT passthru interrupts don't go through here either.
 
> > This may improve performance of emulated devices that go through
> > qemu_set_irq(), eg. virtio devices created with ioeventfd=off or
> > configured by the guest to use LSI interrupts, which aren't really
> > recommended setups.
> 
> LGTM.
> 
> Any figures to share ? 
> 

No. Since performance critical setups don't go through that path,
I didn't try too much.

> C.
> 
> > Signed-off-by: Greg Kurz <groug@kaod.org>
> > ---
> >  hw/intc/spapr_xive_kvm.c |   16 ++--------------
> >  1 file changed, 2 insertions(+), 14 deletions(-)
> > 
> > diff --git a/hw/intc/spapr_xive_kvm.c b/hw/intc/spapr_xive_kvm.c
> > index 08012ac7cd76..69e73552f1ef 100644
> > --- a/hw/intc/spapr_xive_kvm.c
> > +++ b/hw/intc/spapr_xive_kvm.c
> > @@ -354,32 +354,20 @@ static void kvmppc_xive_source_get_state(XiveSource *xsrc)
> >  void kvmppc_xive_source_set_irq(void *opaque, int srcno, int val)
> >  {
> >      XiveSource *xsrc = opaque;
> > -    SpaprXive *xive = SPAPR_XIVE(xsrc->xive);
> > -    struct kvm_irq_level args;
> > -    int rc;
> > -
> > -    /* The KVM XIVE device should be in use */
> > -    assert(xive->fd != -1);
> >  
> > -    args.irq = srcno;
> >      if (!xive_source_irq_is_lsi(xsrc, srcno)) {
> >          if (!val) {
> >              return;
> >          }
> > -        args.level = KVM_INTERRUPT_SET;
> >      } else {
> >          if (val) {
> >              xsrc->status[srcno] |= XIVE_STATUS_ASSERTED;
> > -            args.level = KVM_INTERRUPT_SET_LEVEL;
> >          } else {
> >              xsrc->status[srcno] &= ~XIVE_STATUS_ASSERTED;
> > -            args.level = KVM_INTERRUPT_UNSET;
> >          }
> >      }
> > -    rc = kvm_vm_ioctl(kvm_state, KVM_IRQ_LINE, &args);
> > -    if (rc < 0) {
> > -        error_report("XIVE: kvm_irq_line() failed : %s", strerror(errno));
> > -    }
> > +
> > +    xive_esb_trigger(xsrc, srcno);
> >  }
> 
>