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

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Date:   Thu, 22 Apr 2021 16:51:27 +0100
From:   Andrew Cooper <andrew.cooper3@...rix.com>
To:     Jan Beulich <jbeulich@...e.com>, 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 16:42, Jan Beulich wrote:
> 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.

The trainwreck around gnttab v2 is a mistake I hope we're never going to
make again.  I don't think it's a useful argument here.

The logic is fine.  It's checking for the actual features in the ABI
upon which Linux depends.

Sure - someone could modify Xen to take the feature out, but they'd get
a red wall in CI as they break every Linux kernel released in the past
decade.

~Andrew