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  1. OASIS Virtual I/O Device (VIRTIO) TC
  2. VIRTIO-179

VirtIO spec issue - Available Buffer Notification Suppression

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    Details

    • Type: Bug
    • Status: Deferred
    • Priority: Major
    • Resolution: Unresolved
    • Affects Version/s: virtio 1.1 cs01
    • Fix Version/s: None
    • Labels:
      None
    • Environment:

      "Savir, Gil" <gil.savir@intel.com>

    • Proposal:
      Hide

      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).

      Show
      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).

      Description

      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.

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            • Assignee:
              Unassigned
              Reporter:
              mstsirkin Michael S. Tsirkin
            • Watchers:
              1 Start watching this issue

              Dates

              • Created:
                Updated: