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Subject: Re: [PATCH 0/3] xen: remove some checks for always present Xen
 features
To:     Juergen Gross <jgross@suse.com>
Cc:     Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Stefano Stabellini <sstabellini@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Peter Zijlstra <peterz@infradead.org>,
        linux-kernel@vger.kernel.org, x86@kernel.org,
        xen-devel@lists.xenproject.org
References: <20210422151007.2205-1-jgross@suse.com>
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 <b69df7d3-6fcb-a565-9ec5-a272b6163320@suse.com>
 <08e3fcf1-dabc-c550-f76c-47a78a12274b@suse.com>
 <ee2bc5f9-58ea-bad3-1343-2156d2b80ad3@suse.com>
From:   Jan Beulich <jbeulich@suse.com>
Message-ID: <e293427d-b178-fd22-28bf-e6c6fe7e3f83@suse.com>
Date:   Thu, 22 Apr 2021 17:42:01 +0200
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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