Re: [virtio] Re: [PATCH v8 08/16] packed virtqueues: more efficient virtqueue layout

[Halil Pasic <pasic@linux.vnet.ibm.com>]

On 02/27/2018 03:11 PM, Michael S. Tsirkin wrote:
>> [..]
>>>>> +
>>>>> +\devicenormative{\subsection}{The Virtqueue Descriptor Table}{Basic Facilities of a Virtio Device / Packed Virtqueues / The Virtqueue Descriptor Table}
>>>>> +A device MUST NOT write to a device-readable buffer, and a device SHOULD NOT
>>>>> +read a device-writable buffer.
>>>>> +A device MUST NOT use a descriptor unless it observes
>>>>> +VIRTQ_DESC_F_AVAIL bit in its \field{flags} being changed.
>>>> I don't really understand this. How does the device observe
>>>> the VIRTQ_DESC_F_AVAIL bit being changed?
>>> By reading the descriptor.
>>>
>> :) My point is: to observe a change one usually either needs at
>> least one reading before and at least one reading after the change,
>> or one needs to know that a certain reading means change. The latter
>> is possible if we know that at the beginning of the time frame under
>> consideration (t_0) only a certain set of values,let's say B like before,
>> is possible, and after the change only a certain other set of values
>> let's say A like after, is possible, and A and B are disjunctive (
>> $A \cap B = \emtyset$).
> Well each descriptor is read each time ring wraps around,
> and the bit value changes each time ring wraps around.
> For example device knows it's zero initialized so
> if it reads bit value as 1 it knows the bit value has changed.
> 
> 

Yeah I kind of understand but I would like having a more straightforward
formulation here (than changes).

BTW does this mean that the vhost implementation (that is:

+static bool desc_is_avail(struct vhost_virtqueue *vq,
+			  struct vring_desc_packed *desc)
+{
+	if (vq->used_wrap_counter)
+		if ((desc->flags & DESC_AVAIL) && !(desc->flags & DESC_USED))
+			return true;
+	if (vq->used_wrap_counter == false)
+		if (!(desc->flags & DESC_AVAIL) && (desc->flags & DESC_USED))
+			return true;
+
+	return false;
+}

) is needlessly looking at the 'used' bit? (I think that is the case.)

Bottom line is: I would like avail/used protocol described in a less
ambiguous fashion.

However if I'm the only one who finds this aspect hard to understand,
the problem probably lies with me and not with the text. I can accept
that too.