Re: [Xen-devel] [PATCH] x86/IRQ: make internally used IRQs also honor the pending EOI stack

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Thu, Nov 28, 2019 at 12:03:47PM +0100, Jan Beulich wrote:
> At the time the pending EOI stack was introduced there were no
> internally used IRQs which would have the LAPIC EOI issued from the
> ->end() hook. This had then changed with the introduction of IOMMUs,
> but the interaction issue was presumably masked by
> irq_guest_eoi_timer_fn() frequently EOI-ing interrupts way too early
> (which got fixed by 359cf6f8a0ec ["x86/IRQ: don't keep EOI timer
> running without need"]).
> 
> The problem is that with us re-enabling interrupts across handler
> invocation, a higher priority (guest) interrupt may trigger while
> handling a lower priority (internal) one. The EOI issued from
> ->end() (for ACKTYPE_EOI kind interrupts) would then mistakenly
> EOI the higher priority (guest) interrupt, breaking (among other
> things) pending EOI stack logic's assumptions.

Maybe there's something that I'm missing, but shouldn't hypervisor
vectors always be higher priority than guest ones?

I see there's already a range reserved for high priority vectors
({FIRST/LAST}_HIPRIORITY_VECTOR), what's the reason for hypervisor
interrupts not using this range?

IMO it seems troublesome that pending guests vectors can delay the
injection of hypervisor interrupt vectors.

Thanks, Roger.

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