linux-kernel - Re: [PATCH 2/4] xen: limit grant v2 interface to the v1 functionality

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Date:   Tue, 12 Sep 2017 20:18:12 +0200
From:   Juergen Gross <jgross@...e.com>
To:     Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        linux-kernel@...r.kernel.org, xen-devel@...ts.xenproject.org
Subject: Re: [PATCH 2/4] xen: limit grant v2 interface to the v1 functionality

On 12/09/17 18:21, Boris Ostrovsky wrote:
> On 09/12/2017 12:09 PM, Juergen Gross wrote:
>> On 12/09/17 18:05, Boris Ostrovsky wrote:
>>> On 09/12/2017 11:50 AM, Juergen Gross wrote:
>>>> On 12/09/17 17:44, Boris Ostrovsky wrote:
>>>>> On 09/08/2017 10:48 AM, Juergen Gross wrote:
>>>>>> As there is currently no user for sub-page grants or transient grants
>>>>>> remove that functionality. This at once makes it possible to switch
>>>>>> from grant v2 to grant v1 without restrictions, as there is no loss of
>>>>>> functionality other than the limited frame number width related to
>>>>>> the switch.
>>>>> But isn't that ABI violation? v2 is expected to support this (XSAs
>>>>> notwithstanding)
>>>> No, I don't think so.
>>>>
>>>> The hypervisor still supports it, but the domU (or dom0) isn't required
>>>> to make use of all the features IMHO. Or are you aware of any backend
>>>> querying the grant version of a frontend and acting in another way if v2
>>>> is detected?
>>> I am not aware of any but that doesn't mean that they don't (or won't)
>>> exist.
>> But isn't the frontend the one which is defining what is granted in
>> which way? How should there be an ABI breakage when the frontend just
>> isn't using sub-page or transitive grants?
> 
> People may provide both front and backend drivers and frontends, knowing
> that v2 is available, could decide to use those features.

No, without the functions to use them it will be impossible. So it won't
hit them on a random system by not working, but they would not be able
to load such a driver (same as today without V2 support).

In case they really want it they can send patches for support of subpage
or transient grants. Like they would have to do for complete V2 support
today.


Juergen