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From: Andrew Cooper <andrew.cooper3@citrix.com>
To: Jan Beulich <jbeulich@suse.com>, 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>
Subject: Re: [PATCH 0/3] xen: remove some checks for always present Xen features
Date: Thu, 22 Apr 2021 16:51:27 +0100	[thread overview]
Message-ID: <51e880f0-698e-8208-1a04-fca3942cd6eb@citrix.com> (raw)
In-Reply-To: <e293427d-b178-fd22-28bf-e6c6fe7e3f83@suse.com>

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



  parent reply	other threads:[~2021-04-22 15:52 UTC|newest]

Thread overview: 17+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-04-22 15:10 Juergen Gross
2021-04-22 15:10 ` [PATCH 1/3] xen: check required " Juergen Gross
2021-04-22 15:26   ` Stefano Stabellini
2021-04-22 15:32     ` Juergen Gross
2021-05-10 12:11   ` Boris Ostrovsky
2021-05-10 13:21     ` Juergen Gross
2021-04-22 15:10 ` [PATCH 2/3] xen: assume XENFEAT_mmu_pt_update_preserve_ad being set for pv guests Juergen Gross
2021-04-22 15:10 ` [PATCH 3/3] xen: assume XENFEAT_gnttab_map_avail_bits " Juergen Gross
2021-04-22 15:16 ` [PATCH 0/3] xen: remove some checks for always present Xen features Jan Beulich
2021-04-22 15:17   ` Juergen Gross
2021-04-22 15:23     ` Jan Beulich
2021-04-22 15:28       ` Juergen Gross
2021-04-22 15:42         ` Jan Beulich
2021-04-22 15:49           ` Juergen Gross
2021-04-22 15:51           ` Andrew Cooper [this message]
2021-05-10  7:34 ` Juergen Gross
2021-05-10 11:31   ` Peter Zijlstra

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