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

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Date:   Tue, 12 Sep 2017 12:21:30 -0400
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Juergen Gross <jgross@...e.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 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.

-boris