Re: [virtio-comment] Re: virtio-sound linux driver conformance to spec

[Paolo Bonzini <pbonzini@redhat.com>]

On 9/19/23 02:35, Anton Yakovlev wrote:
> If the Linux virtio sound driver violates a specification, then there 
> must be
> a conformance statement that the driver does not follow. As far as I know,
> there is no such thing at the moment.

There is one in 2.7.13.3: "The device MAY access the descriptor chains 
the driver created and the memory they refer to immediately"

And likewise for packed virtqueues in 2.8.21.1: "The device MAY access 
the descriptor and any following descriptors the driver created and the 
memory they refer to immediately"

I think it's a mistake to use MAY here, as opposed to "may".  This is 
not an optional feature, it's a MUST NOT requirement on the driver's 
part that should be in 2.7.13.3.1 and 2.8.21.1.1.

This does not prevent the virtio-snd spec from overriding this.  If an 
override is desirable (for example because other hardware behaves like 
this), there should be a provision in 2.7.13.3.1 and 2.8.21.1.1.  For 
example:

2.7.13.3.1 Unless the device specification specifies otherwise, the 
driver MUST NOT write to the descriptor chains and the memory they refer 
to, between the /idx/ update and the time the device places the driver 
on the used ring.

2.8.21.1.1 "Unless the device specification specifies otherwise, the 
driver MUST NOT write to the descriptor, to any following descriptors 
the driver created, nor to the memory the refer to, between the /flags/ 
update and the time the device places the driver on the used ring.


In the virtio-snd there would be a normative statement like

5.14.6.8.1.1  The device MUST NOT read from available device-readable 
buffers beyond the first buffer_bytes / period_bytes periods.

5.14.6.8.1.2  The driver MAY write to device-readable buffers beyond the 
first buffer_bytes / period_bytes periods, even after offering them to 
the device.



As an aside, here are two other statements that have a similar issue:

- 2.6.1.1.2 "the driver MAY release any resource associated with that
virtqueue" (instead 2.6.1.1.1 should have something like "After a queue 
has been reset by the driver, the device MUST NOT access any resource 
associated with a virtqueue").

- 2.7.5.1 "[the device] MAY do so for debugging or diagnostic purposes" 
(this is not normative and can be just "may")


Thanks,

Paolo