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

[Cédric Le Goater]

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.

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

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

C.

> Signed-off-by: Greg Kurz <address@hidden>
> ---
>  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);
>  }