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

[Christoffer Dall <cdall@kernel.org>]

On Thu, Mar 08, 2018 at 09:49:43AM +0000, Marc Zyngier wrote:
> [updated Christoffer's email address]
> 
> Hi Shunyong,
> 
> On 08/03/18 07:01, Shunyong Yang wrote:
> > When resampling irqfds is enabled, level interrupt should be
> > de-asserted when resampling happens. On page 4-47 of GIC v3
> > specification IHI0069D, it said,
> > "When the PE acknowledges an SGI, a PPI, or an SPI at the CPU
> > interface, the IRI changes the status of the interrupt to active
> > and pending if:
> > • It is an edge-triggered interrupt, and another edge has been
> > detected since the interrupt was acknowledged.
> > • It is a level-sensitive interrupt, and the level has not been
> > deasserted since the interrupt was acknowledged."
> > 
> > GIC v2 specification IHI0048B.b has similar description on page
> > 3-42 for state machine transition.
> > 
> > When some VFIO device, like mtty(8250 VFIO mdev emulation driver
> > in samples/vfio-mdev) triggers a level interrupt, the status
> > transition in LR is pending-->active-->active and pending.
> > Then it will wait resampling to de-assert the interrupt.
> > 
> > Current design of lr_signals_eoi_mi() will return false if state
> > in LR is not invalid(Inactive). It causes resampling will not happen
> > in mtty case.
> 
> Let me rephrase this, and tell me if I understood it correctly:
> 
> - A level interrupt is injected, activated by the guest (LR state=active)
> - guest exits, re-enters, (LR state=pending+active)
> - guest EOIs the interrupt (LR state=pending)
> - maintenance interrupt
> - we don't signal the resampling because we're not in an invalid state
> 
> Is that correct?
> 
> That's an interesting case, because it seems to invalidate some of the 
> optimization that went in over a year ago.
> 
> 096f31c4360f KVM: arm/arm64: vgic: Get rid of MISR and EISR fields
> b6095b084d87 KVM: arm/arm64: vgic: Get rid of unnecessary save_maint_int_state
> af0614991ab6 KVM: arm/arm64: vgic: Get rid of unnecessary process_maintenance operation
> 
> We could compare the value of the LR before the guest entry with
> the value at exit time, but we still could miss it if we have a
> transition such as P+A -> P -> A and assume a long enough propagation
> delay for the maintenance interrupt (which is very likely).
> 
> In essence, we have lost the benefit of EISR, which was to give us a
> way to deal with asynchronous signalling.
> 

I don't understand why EISR gives us anything beyond looking at the LR
and evaluating if the state is 00.  My reading of the spec is that the
EISR is merely a shortcut to knowing the state of the LRs but contains
not record or information beyond what you can read from the LRs.

What am I missing?

Thanks,
-Christoffer