Re: [RFC PATCH] KVM: arm/arm64: vgic: change condition for level interrupt resampling

[Marc Zyngier]

On Sun, 11 Mar 2018 01:55:08 +0000
Christoffer Dall <cdall@xxxxxxxxxx> wrote:

> On Sat, Mar 10, 2018 at 12:20 PM, Marc Zyngier <marc.zyngier@xxxxxxx> wrote:
> > On Fri, 09 Mar 2018 21:36:12 +0000,
> > Christoffer Dall wrote:  
> >>
> >> On Thu, Mar 08, 2018 at 05:28:44PM +0000, Marc Zyngier wrote:  
> >> > I'd be more confident if we did forbid P+A for such interrupts
> >> > altogether, as they really feel like another kind of HW interrupt.  
> >>
> >> How about a slightly bigger hammer:  Can we avoid doing P+A for level
> >> interrupts completely?  I don't think that really makes much sense, and
> >> I think we simply everything if we just come back out and resample the
> >> line.  For an edge, something like a network card, there's a potential
> >> performance win to appending a new pending state, but I doubt that this
> >> is the case for level interrupts.  
> >
> > I started implementing the same thing yesterday. Somehow, it feels
> > slightly better to have the same flow for all level interrupts,
> > including the timer, and we only use the MI on EOI as a way to trigger
> > the next state of injection. Still testing, but looking good so far.
> >
> > I'm still puzzled that we have this level-but-not-quite behaviour for
> > VFIO interrupts. At some point, it is going to bite us badly.
> >  
> 
> Where is the departure from level-triggered behavior with VFIO?  As
> far as I can tell, the GIC flow of the interrupts will be just a level
> interrupt, 

The GIC is fine, I believe. What is not exactly fine is the signalling
from the device, which will never be dropped until the EOI has been
detected.

> but we just need to make sure the resamplefd mechanism is
> supported for both types of interrupts.  Whether or not that's a
> decent mechanism seems orthogonal to me, but that's a discussion for
> another day I think.

Given that VFIO is built around this mechanism, I don't think we have a
choice but to support it. Anyway, I came up with the following patch,
which I tested on Seattle with mtty. It also survived my usual
hammering of cyclictest, hackbench  and bulk VM installs.

Shunyong, could you please give it a go?

Thanks,

	M.