Re: [PATCH linux-2.6.18] xen: mark pvscsi frontend request consumed only after last read

[Juergen Gross <jgross@suse.com>]

On 02/02/2015 08:52 AM, Jan Beulich wrote:
>>>> On 30.01.15 at 14:51, <"jgross@suse.com".non-mime.internet> wrote:
>> A request in the ring buffer mustn't be read after it has been marked
>> as consumed. Otherwise it might already have been reused by the
>> frontend without violating the ring protocol.
>
> This is irrelevant, as the ->req_cons is a backend private field (if it
> was a shared one, a barrier would have been needed between
> copying and updating that field).

Hmm, you are right. Interesting, I've always thought req_cons would be
used by the frontend, too.

Thanks for updating my protocol know-how. :-)

>
>> To avoid inconsistencies in the backend only work on a private copy
>> of the request. This will ensure a malicious guest not being able to
>> bypass consistency checks of the backend by modifying an active
>> request.
>
> Hence - also considering with my earlier reply - I don't view the change
> as necessary.

Agreed.


Juergen