Subject: Re: [PATCH 2/4] xen: limit grant v2 interface to the v1 functionality
To: Juergen Gross <jgross@suse.com>, linux-kernel@vger.kernel.org,
        xen-devel@lists.xenproject.org
References: <20170908144849.2958-1-jgross@suse.com>
 <20170908144849.2958-3-jgross@suse.com>
 <f9cc6652-c524-bf4f-73f0-494d16deedf0@oracle.com>
 <8c87dc06-bbc8-5279-d135-d3351032913d@suse.com>
 <2777c0f8-7555-fbf3-6e18-81cea2fd1a39@oracle.com>
 <7dd99241-f771-9fa2-8a6a-ce019dbe08b6@suse.com>
 <46a17191-916a-e039-7e2f-a197e6783fe4@oracle.com>
 <258bc8dd-c475-bd5d-dffc-8c77fbe3cca3@suse.com>
From: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Message-ID: <9e8e497a-49d1-6299-8101-dd1123f81fee@oracle.com>
Date: Wed, 13 Sep 2017 09:22:23 -0400
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101
 Thunderbird/45.8.0
MIME-Version: 1.0
In-Reply-To: <258bc8dd-c475-bd5d-dffc-8c77fbe3cca3@suse.com>
Content-Type: text/plain; charset=windows-1252
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1931
Lines: 44

On 09/12/2017 02:18 PM, Juergen Gross wrote:
> 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.

I don't follow this. Which functions? The ones this patch is removing?

-boris

>  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
>