Re: PCI Driver Requirements: ISR status capability

On Fri, May 06, 2016 at 09:28:20AM +0200, Ladi Prosek wrote:
> Hello virtio-comment,
> 
> This is a comment on the wording of "4.1.4.5.2 Driver Requirements:
> ISR status capability" in virtio 1.0 spec CS 04, copied below.
> 
> Reading ISR status if MSI is enabled is not guaranteed to give any
> meaningful results and the behavior should be specified as "undefined"
> and/or the requirement level should be "MUST NOT". The use of "SHOULD
> NOT" indicates optionality and the sentence may be interpreted as a
> perf optimization suggestion (no need to read ISR status because the
> MSI interrupt provides the required information), giving the
> impression that there are circumstances when accessing the capability
> is in fact useful.
> 
> 4.1.4.5.2 Driver Requirements: ISR status capability
> 
> If MSI-X capability is enabled, the driver SHOULD NOT access ISR
> status upon detecting a Queue Interrupt.
> 
> Thanks!
> Ladi

I think I agree. I would add something like:

If MSI-X capability is enabled, the driver MUST ignore
bit 0 (Queue Interrupt) in the ISR register.

-- 
MST