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* [virtio-comment] VirtIO spec issue - Available Buffer Notification Suppression
@ 2019-01-31 13:16 Savir, Gil
  2019-02-11  6:58 ` Michael S. Tsirkin
  0 siblings, 1 reply; 6+ messages in thread
From: Savir, Gil @ 2019-01-31 13:16 UTC (permalink / raw)
  To: virtio-comment@lists.oasis-open.org; +Cc: Elmaleh, Liron

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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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^ permalink raw reply	[flat|nested] 6+ messages in thread

end of thread, other threads:[~2019-02-26 20:30 UTC | newest]

Thread overview: 6+ messages (download: mbox.gz follow: Atom feed
-- links below jump to the message on this page --
2019-01-31 13:16 [virtio-comment] VirtIO spec issue - Available Buffer Notification Suppression Savir, Gil
2019-02-11  6:58 ` Michael S. Tsirkin
2019-02-11  7:02   ` Michael S. Tsirkin
2019-02-21  8:57     ` Savir, Gil
2019-02-22  1:28       ` Michael S. Tsirkin
2019-02-26 20:30         ` Michael S. Tsirkin

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