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Subject: 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 --------------------------------------------------------------------- This e-mail and any attachments may contain confidential material for |
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