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  • 1.  PCI Driver Requirements: ISR status capability

    Posted 05-06-2016 07:28
    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



  • 2.  Re: PCI Driver Requirements: ISR status capability

    Posted 05-06-2016 12:08
    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



  • 3.  Re: PCI Driver Requirements: ISR status capability

    Posted 05-06-2016 12:08
    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


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