linux-kernel - Re: [PATCH 0/3] xen: remove some checks for always present Xen features

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Date:   Thu, 22 Apr 2021 17:42:01 +0200
From:   Jan Beulich <jbeulich@...e.com>
To:     Juergen Gross <jgross@...e.com>
Cc:     Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        xen-devel@...ts.xenproject.org
Subject: Re: [PATCH 0/3] xen: remove some checks for always present Xen
 features

On 22.04.2021 17:28, Juergen Gross wrote:
> On 22.04.21 17:23, Jan Beulich wrote:
>> On 22.04.2021 17:17, Juergen Gross wrote:
>>> On 22.04.21 17:16, Jan Beulich wrote:
>>>> On 22.04.2021 17:10, Juergen Gross wrote:
>>>>> Some features of Xen can be assumed to be always present, so add a
>>>>> central check to verify this being true and remove the other checks.
>>>>>
>>>>> Juergen Gross (3):
>>>>>     xen: check required Xen features
>>>>>     xen: assume XENFEAT_mmu_pt_update_preserve_ad being set for pv guests
>>>>>     xen: assume XENFEAT_gnttab_map_avail_bits being set for pv guests
>>>>
>>>> I wonder whether it's a good idea to infer feature presence from
>>>> version numbers. If (at some point in the past) you had inferred
>>>> gnttab v2 being available by version, this would have been broken
>>>> by its availability becoming controllable by a command line option
>>>> in Xen.
>>>
>>> I'm testing the feature to be really present when booting and issue a
>>> message if it is not there.
>>
>> And how does this help if the feature really isn't there yet other code
>> assumes it is?
> 
> Did you look at the features I'm testing?

I did, yes.

> Those are really just low
> level additions I can't imagine will ever be removed again.

I don't expect them to be removed. But I don't think the people having
contributed gnttab v2 expected any such for it, either.

Jan