VirtIO spec issue - Available Buffer Notification Suppression

Hi,

 

If VIRTIO_F_EVENT_IDX feature bit is negotiated, then Available Buffer Notification Suppression mechanism used is
avail event (not flags).

The spec (both v1.0 / v1.1-draft) states that the device
MAY use this mechanism (Paragraph 2.4.9.2 / 2.6.10.2 respectively).

This statement implies that the device may choose
not to use this suppression mechanism (even if VIRTIO_F_EVENT_IDX was negotiated).

 

However – there’s no way for the device to inform the driver that he is not using
avail_event. 

As consequence, since there will be a default value in
avail_event (probably 0x0), then the driver will always assume that it has to send notify “once-per ring”.

This will render performance futile, or force the device to actively update
avail_event.

 

Is there a way for the device to inform the driver that he is not using
avail_event (and I missed it)?

 

If yes, than my apologies for wasting your time.

If no, then I suggest one of the following:

·       
 Either, to change the “MAY” (referred above) to “MUST”,

·       
Or, to add way for the device to inform the driver that he is not using
avail_event (flag /certain reserved value in avail_event /other mechanism).

 

Thanks,

Gil Savir

Intel Corporation

 

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