Re: [virtio] [PATCH] used ring: define the meaning and requirements of the len field.

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

"Michael S. Tsirkin" <mst@redhat.com> writes:
> On Fri, Mar 20, 2015 at 11:48:55AM +1030, Rusty Russell wrote:
>> +\devicenormative{\subsubsection}{Virtqueue Notification Suppression}{Basic Facilities of a Virtio Device / Virtqueues / The Virtqueue Used Ring}
>> +
>> +The device MUST set \field{len} prior to updating the used \field{idx}.
>> +
>> +The device MUST write at least \field{len} bytes to descriptor,
>> +beginning at the first device-writable buffer,
>> +prior to updating the used \field{idx}.
>> +
>> +The device MAY write more than \field{len} bytes to descriptor.
>> +
>> +\begin{note}
>> +There are potential error cases where a device might not know what
>> +parts of the buffers have been written.  This is why \field{len} is
>> +permitted to be an underestimate: that's preferable to the driver believing
>> +that uninitialized memory has been overwritten when it has not.
>> +\end{note}
>> +
>> +\drivernormative{\subsubsection}{Virtqueue Notification Suppression}{Basic Facilities of a Virtio Device / Virtqueues / The Virtqueue Used Ring}
>> +
>> +The driver MUST NOT make assumptions about data in device-writable buffers
>> +beyond the first \field{len} bytes, and SHOULD ignore this data.
>> +
>>  \subsection{Virtqueue Notification Suppression}\label{sec:Basic Facilities of a Virtio Device / Virtqueues / Virtqueue Notification Suppression}
>>  
>>  The device can suppress notifications in a manner analogous to the way
>
> We know legacy devices don't follow this, so we also need some text in
> the legacy sections to document the differences.

Indeed, but it was specific to block devices.

It may be worth noting that older block devices would set erroneously
set a length corresponding to the entire buffer length, and thus legacy
drivers SHOULD rely on the status byte and ignore used.len?

Cheers,
Rusty.