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

[David Gibson]

On Tue, Nov 19, 2019 at 09:15:52AM +0100, Cédric Le Goater 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. 
> 
> >> Any figures to share ? 
> 
> I am torturing Greg to have numbers :) but he resisted well.
> 
> >> C.
> >>
> >>> Signed-off-by: Greg Kurz <address@hidden>
> 
> Let's move on.
> 
> Reviewed-by: Cédric Le Goater <address@hidden>

Works for me.  Applied.

-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson

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