Re: (VIRTIO-13) Remove VIRTIO_F_NOTIFY_ON_EMPTY?

[Rusty Russell <rusty@au1.ibm.com>]

Rusty Russell <rusty@au1.ibm.com> writes:
> OASIS Issues Tracker <workgroup_mailer@lists.oasis-open.org> writes:
>>  Remove VIRTIO_F_NOTIFY_ON_EMPTY?
>> ---------------------------------
>>
>>                  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:

This is updated on the more recent draft, using an explicit legacy
section:

commit 65b90ebe8be6a55c4c3c5033d597a6c3de87adf8
Author: Rusty Russell <rusty@au1.ibm.com>
Date:   Mon Sep 9 16:55:06 2013 +0930

    Remove VIRTIO_F_NOTIFY_ON_EMPTY
    
    As per http://tools.oasis-open.org/issues/browse/VIRTIO-13
    
    (Includes le8 -> u8 fix)
    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 9b0bbca..aea3b3f 100644
--- a/virtio-v1.0-wd01-part1-specification.txt
+++ b/virtio-v1.0-wd01-part1-specification.txt
@@ -1361,9 +1361,7 @@ 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.
+packets.
 
 The normal behavior in this interrupt handler is to retrieve and
 new descriptors from the used ring and free the corresponding
@@ -2568,18 +2566,7 @@ contents of the event field. The following events are defined:
 2.5. Reserved Feature Bits
 =========================
 
-Currently there are five 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 four device-independent feature bits defined:
 
   VIRTIO_F_ANY_LAYOUT (27) This feature indicates that the device accepts arbitrary
   descriptor layouts, as described in Section "2.1.4.2. Message Framing".
@@ -2607,6 +2594,22 @@ In addition, bit 30 is used by qemu's implementation to check for experimental
 early versions of virtio which did not perform correct feature negotiation,
 and should not be used.
 
+2.5.1 Legacy Interface: Reserved Feature Bits
+--------------------------------------------
+
+Legacy or transitional devices may offer the following:
+
+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.
+
 2.6. virtio_ring.h
 =================