Re: [Xen-devel] [PATCH] x86/hvm: more sure APIC assist is aborted if guest EOIs APIC

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 18.01.18 at 12:21, <Paul.Durrant@xxxxxxxxxx> wrote:
> Actually reading through the code and the spec. again, I'm not convinced by 
> the logic even with the EOI fix. It looks to me like vlapic_has_pending_irq() 
> could leave an existing assist in place and return a higher priority irr 
> value. This guest would then service the higher priority interrupt, skip the 
> EOI because the assist bit is set, but then the next call to 
> vlapic_has_pending_irq() would erroneously clear the lower priority vector 
> from the ISR because we originally latched the lower priority vector.
> As the spec. points out... only the first EOI can avoided, thus I'm not sure 
> trying to track the vector in the viridian code is worth it. It looks like 
> the correct thing to do as test whether an EOI was avoided and then clear the 
> highest priority vector in the ISR (since, if we hadn't squashed it, that 
> would have been the first EOI). We can test when an EOI is avoided because 
> the bit in the shared page would have transitioned from 1 to 0.

Sounds plausible, looking forward to a patch.

Jan


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