Re: [Xen-devel] [Draft F] Xen on ARM vITS Handling

[Ian Campbell <ian.campbell@xxxxxxxxxx>]

On Fri, 2015-06-12 at 13:55 -0400, Julien Grall wrote:
> 
> On 12/06/2015 10:24, Ian Campbell wrote:
> > On Fri, 2015-06-12 at 09:32 -0400, Julien Grall wrote:
> >>
> >> On 12/06/2015 09:16, Ian Campbell wrote:
> >>> On Fri, 2015-06-12 at 09:09 -0400, Julien Grall wrote:
> >>>> Hi Ian,
> >>>>
> >>>> On 12/06/2015 04:52, Ian Campbell wrote:
> >>>>> On Fri, 2015-06-12 at 14:07 +0530, Vijay Kilari wrote:
> >>>>> So pLPIs must be routed at device assignment time because in the vLPI
> >>>>> configuration table trap there is no mapping back to a single pLPI.
> >>>>
> >>>> I just remembered the exact reason that made use to differ SPI enabling.
> >>>
> >>> I can't parse this sentence, differ how?
> >>
> >> deferring sorry.
> >>
> >>>
> >>>> When the device is assigned, the domain VCPUs are still down (even 
> >>>> VCPU0).
> >>>>
> >>>> If we receive an interrupt before the VCPU0 is unpaused, the interrupt
> >>>> will be lost. Same if the interrupt is not yet configured (i.e before
> >>>> the vITS setup correctly the table) with your proposal.
> >>>
> >>> Is this any different to booting with the ITT not setup?
> >>
> >> I don't understand your question.
> >
> > During boot the ITT is not configured and a spurious event will go
> > undelivered to an LPI then too, even on native.
> 
> It's different. In the case of native, the event is not recorded by the 
> ITS so it can fire up again later when the ITT is setup (for instance 
> because the device has been reset).
> 
> With your proposal, the interrupt will go in "Active" state (from the 
> CPU POV see 4.8.3) in the GIC. If you don't EOI it, it will never fire 
> again when the guest has setup the vITT.

Our other option is to EIO it, which has its own issues (namely possible
interrupt storms).

For PCI passthrough it may be that we can mitigate that some what by
resetting the device ourselves or something.

> Although, the device won't know where it has to write the event ID (i.e 
> in GITS_TRANSLATER) because it should not have been configured. So it 
> will get ignored, right?

I think so, yes.

> >
> >>>> This could happen when the device is not quiescent. We had this issue on
> >>>> the vexpress at boot time when the network card was trying to send an
> >>>> interrupt before DOM0 is setup.
> >>>
> >>> I don't fully understand the issue you are trying to describe, but do
> >>> you want to propose a change to the spec?
> >>
> >> I actually don't know how to modify it. So it's an open question.
> >
> > For SPI too, or just for LPI?
> 
> Only LPI.

How is it fixed for SPI?

> >> vgic_vcpu_inject_irq doesn't queue the interrupt if a VCPU is down. I
> >> think this is because the state of the VCPU wouldn't be correct.
> >>
> >> The process would be something like:
> >>
> >>       - Creation of the domain
> >>    => All vCPUs are down
> >>
> >>       - Device is assigned to the guest
> >>           => Enable physical LPIs
> >>
> >>       * physical LPI is received *
> >>    => Will be ignored and not EOIed (VCPU0 is down)
> >>           => The LPI will never fired again during the guest life
> >>
> >>       -  Domain is started by the toolstack
> >>            => VCPU0 is online
> >
> > Is it sufficient to queue interrupts even for VCPUs which are down? How
> > does the lack of a vITT entry when this interrupt occurred affect this?
> 
> Well, in this case we don't know on which vLPI we have to inject it. But 
> as said above, I guess we don't care if we ensure that the device can't 
> send an event (by ensuring that the device doesn't know the 
> GITS_TRANSLATER address is).

Yes, I think that works.

I'm not sure where to spell that out though.

Ian.



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