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

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Message-ID: <0ea7c145-9a86-f002-75d0-bc6b8be72867@oracle.com>
Date:   Wed, 13 Sep 2017 09:50:15 -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 09/13/2017 09:38 AM, Juergen Gross wrote:
> On 13/09/17 15:22, Boris Ostrovsky wrote:
>> 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?
> Yes, just after having been added one patch earlier.
>
> Right now the Linux kernel doesn't support grant V2 at all. So there
> surely is no driver making use of V2 features right now.
>
> Ican merge patches 1 and 2 in case you want. I just thought a pure
> revert of the former V2 remove patch would be easier to review,
> taking into account that the former V2 support was working in
> production environments (and even back then there was no user of
> sub-page or transient grants).

No, I don't have problems with *how* you are doing this (revert fully
first and then remove).

I am just not sure that removing these functions is the way to go
because we are ending up with partial implementation of v2. The fact
that noone is/has been using these features is IMO not particularly
relevant.

If these two were optional features then it would have been reasonable
to drop them.

-boris