Re: [virtio-comment] Question regarding VIRTIO_RING_F_EVENT_IDX

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

Hi Haggai,

        Indeed, excellent catch!

I've boiled down your feedback to the patch below, which we'll discuss
at the next meeting.

Thanks,
Rusty.

diff --git a/content.tex b/content.tex
index 374bc02..f048a88 100644
--- a/content.tex
+++ b/content.tex
@@ -425,7 +425,7 @@ written by the driver and read by the device.
 The “idx” field indicates where the driver would put the next descriptor
 entry in the ring (modulo the queue size). This starts at 0, and increases.
 
-If the VIRTIO_RING_F_INDIRECT_DESC feature bit is not negotiated, the
+If the VIRTIO_RING_F_EVENT_IDX feature bit is not negotiated, the
 “flags” field offers a crude interrupt control mechanism.  The driver
 MUST set this to 0 or 1: 1 indicates that the device SHOULD NOT send
 an interrupt when it consumes a descriptor chain from the available
@@ -473,7 +473,7 @@ for drivers using untrusted buffers: if you do not know exactly
 how much has been written by the device, you usually have to zero
 the buffer to ensure no data leakage occurs.
 
-If the VIRTIO_RING_F_INDIRECT_DESC feature bit is not negotiated, the
+If the VIRTIO_RING_F_EVENT_IDX feature bit is not negotiated, the
 “flags” field offers a crude interrupt control mechanism.  The driver
 MUST initialize this to 0, the device MUST set this to 0 or 1: 1
 indicates that the driver SHOULD NOT send an notification when it adds

Haggai Eran <haggaie@mellanox.com> writes:
> Hi,
>
> In Section 2.4.5, "The Virtqueue Available Ring", it says:
>
>> If the VIRTIO_RING_F_INDIRECT_DESC feature bit is not negotiated, the “flags” field offers a crude interrupt
>> control mechanism. The driver MUST set this to 0 or 1: 1 indicates that the device SHOULD NOT
>> send an interrupt when it consumes a descriptor chain from the available ring. The device MUST ignore the
>> used_event value in this case.
>
> Shouldn't it be VIRTIO_RING_F_EVENT_IDX instead of
> VIRTIO_RING_F_INDIRECT_DESC?
> The next paragraph says:
>
>> Otherwise, if the VIRTIO_RING_F_EVENT_IDX feature bit is negotiated, the driver MUST set the "flags" field
>> to 0, and use the “used_event” field in the used ring instead. The driver can ask the device to delay interrupts
>> until an entry with an index specified by the “used_event” field is written in the used ring (equivalently, until
>> the idx field in the used ring will reach the value used_event + 1).
>
> The same question also applies to Section 2.4.6.
>
> Thanks,
> Haggai
>
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