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

[Greg Kurz <groug@kaod.org>]

On Tue, 19 Nov 2019 09:15:52 +0100
Cédric Le Goater <clg@kaod.org> wrote:

> On 19/11/2019 01:47, David Gibson wrote:
> > On Mon, Nov 18, 2019 at 04:37:16PM +0100, Cédric Le Goater 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.
> >>
> >> 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.
> > 
> > Ok, between the comments above and this, I'm not sure if this is ready
> > to merge or not.
> 
> I think it is. 
> 
> With this change, we are loosing a check on passthrough interrupts but 
> I am not sure how critical this is given that QEMU can anyhow bypass 
> KVM and trigger the interrupt using a store on the ESB page. 
> 

True. Thinking a bit more about this: nothing prevents such a store to
be the result of a bug somewhere else in QEMU, eg. some dangling pointer
with the same value, in a much easier way than doing the KVM ioctl. Is
it a concern we should take into account ?

> >> Any figures to share ? 
> 
> I am torturing Greg to have numbers :) but he resisted well.
> 

Maybe a _liquid_ bribe or two can be convincing enough :-)

> >> C.
> >>
> >>> Signed-off-by: Greg Kurz <groug@kaod.org>
> 
> Let's move on.
> 
> Reviewed-by: Cédric Le Goater <clg@kaod.org>
> 
> C.
> 
> >>> ---
> >>>  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);
> >>>  }
> >>
> >>
> > 
>