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OASIS Issues Tracker <workgroup_mailer@lists.oasis-open.org> writes:
> ---------------------------------
>                  Key: VIRTIO-13
>                  URL: http://tools.oasis-open.org/issues/browse/VIRTIO-13
>              Project: OASIS Virtual I/O Device (VIRTIO) TC
>           Issue Type: Improvement
>             Reporter: Rusty Russell
> Notify on empty means that the device interrupts even if it wouldn't normally, in the case where a normally-full virtqueue is empty.
> ISTR this was a performance hack; it's no longer used in Linux anyway, and performance issues may have been due to a race which sometimes lost interrupts :)

Which would look something like this:

commit 3a4ec12fdf041e2d55c24d4aacfbba829bc228f8
Author: Rusty Russell <rusty@au1.ibm.com>
Date:   Mon Aug 19 18:00:35 2013 +0930

    As per http://tools.oasis-open.org/issues/browse/VIRTIO-13
    Signed-off-by: Rusty Russell <rusty@au1.ibm.com>

diff --git a/virtio-v1.0-wd01-part1-specification.txt b/virtio-v1.0-wd01-part1-specification.txt
index 3e311ec..353ef44 100644
--- a/virtio-v1.0-wd01-part1-specification.txt
+++ b/virtio-v1.0-wd01-part1-specification.txt
@@ -1302,9 +1302,7 @@ the different features the driver negotiated.
 Often a driver will suppress transmission interrupts using the
 VRING_AVAIL_F_NO_INTERRUPT flag (see 2.4.2 Receiving Used Buffers From
 The Device) and check for used packets in the transmit path of following 
-packets. However, it will still receive interrupts if the 
-VIRTIO_F_NOTIFY_ON_EMPTY feature is negotiated, indicating that 
-the transmission queue is completely emptied.
 The normal behavior in this interrupt handler is to retrieve and 
 new descriptors from the used ring and free the corresponding 
@@ -2498,18 +2496,7 @@ contents of the event field. The following events are defined:
 2.6 Reserved Feature Bits
-Currently there are four device-independent feature bits defined:
-  VIRTIO_F_NOTIFY_ON_EMPTY (24) Negotiating this feature 
-  indicates that the driver wants an interrupt if the device runs 
-  out of available descriptors on a virtqueue, even though 
-  interrupts are suppressed using the VRING_AVAIL_F_NO_INTERRUPT 
-  flag or the used_event field. An example of this is the 
-  networking driver: it doesn't need to know every time a packet 
-  is transmitted, but it does need to free the transmitted 
-  packets a finite time after they are transmitted. It can avoid 
-  using a timer if the device interrupts it when all the packets 
-  are transmitted.
+Currently there are three device-independent feature bits defined:
   VIRTIO_F_ANY_LAYOUT (27) This feature indicates that the device accepts arbitrary
   descriptor layouts, as described in Section FIXME.

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