Re: [PATCHv1 0/2] xen/grant-table: fix PV guests and remove V2 support

[David Vrabel <david.vrabel@citrix.com>]

On 04/07/14 13:01, Paul Durrant wrote:
>> -----Original Message-----
>> From: David Vrabel
>> Sent: 04 July 2014 12:55
>> To: Paul Durrant; xen-devel@lists.xenproject.org
>> Cc: Boris Ostrovsky
>> Subject: Re: [Xen-devel] [PATCHv1 0/2] xen/grant-table: fix PV guests and
>> remove V2 support
>>
>> On 04/07/14 11:59, Paul Durrant wrote:
>>>> -----Original Message-----
>>>> From: xen-devel-bounces@lists.xen.org [mailto:xen-devel-
>>>> bounces@lists.xen.org] On Behalf Of David Vrabel
>>>> Sent: 02 July 2014 11:25
>>>> To: xen-devel@lists.xenproject.org
>>>> Cc: Boris Ostrovsky; David Vrabel
>>>> Subject: [Xen-devel] [PATCHv1 0/2] xen/grant-table: fix PV guests and
>>>> remove V2 support
>>>>
>>>> The first patch fixes a logn standing issue where the map/unmap of
>>>> grant table frames by PV guests was unsafe and could BUG() or have
>>>> other bad behaviour.  This was caused by calling function that were
>>>> not permitted from atomic contexts.
>>>>
>>>> The second patch removes all V2 table code since it is unused.
>>>>
>>>
>>> How confident are you of that assertion?
>>
>> This is removing the /Linux/ code which has been using V1 only for a
>> while now.
>>
> 
> If you believe that the API is likely to wither then that seems
> reasonable. I would have thought the code was unlikely to see much
> change though and it seems a shame to remove an interface to something
> that it still there in Xen... I guess *someone* may want to use it in
> future; particularly the copy-only grants could be useful for something
> like IDC (if the backend was unprivileged).

It's already withering. For example PVH doesn't support V2 as it doesn't
handle mapping/populating the status frames.  And fixing a long standing
bug (see patch 1/2) was made more complicated by having to fix up the
(unused) code for dealing with the status frames.

And as an aside, I'm not convinced that copy-only grants are what's
needed for IDC with untrusted backend.  I think revocable grants are
required for efficient usage of shared rings.

David