From: Andrew Cooper <andrew.cooper3@citrix.com>
To: Jan Beulich <jbeulich@suse.com>
Cc: Stefano Stabellini <sstabellini@kernel.org>, Wei Liu <wl@xen.org>,
Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
George Dunlap <George.Dunlap@eu.citrix.com>,
Tim Deegan <tim@xen.org>, Ian Jackson <ian.jackson@eu.citrix.com>,
Eslam Elnikety <elnikety@amazon.com>,
Julien Grall <julien.grall@arm.com>,
Anthony PERARD <anthony.perard@citrix.com>,
xen-devel@lists.xenproject.org,
David Woodhouse <dwmw@amazon.co.uk>
Subject: Re: [Xen-devel] [PATCH] evtchn: make support for different ABIs tunable
Date: Wed, 7 Aug 2019 16:57:32 +0100 [thread overview]
Message-ID: <d2ff9dcf-233b-ac02-1034-310f97e2c215@citrix.com> (raw)
In-Reply-To: <057d72ef-bbec-0e91-b79b-3ebe6da35ba9@suse.com>
On 07/08/2019 16:08, Jan Beulich wrote:
> On 07.08.2019 17:00, Andrew Cooper wrote:
>> On 07/08/2019 15:30, Jan Beulich wrote:
>>> On 07.08.2019 15:41, Andrew Cooper wrote:
>>>> Furthermore, if there is this problem for event channels, then
>>>> there is
>>>> almost certainly a related problem for grant tables.
>>>>
>>>> The control in Xen should be expressed in a positive form, or the
>>>> logic
>>>> will become a tangle. It should be a bit permitting the use of the
>>>> FIFO
>>>> ABI, rather than a bit saying "oh actually, you can't use that".
>>>>
>>>> That said, it might be easier to declare FIFO to be "event channel
>>>> v2",
>>>> and specify max_{grant,evntchn}_ver instead.
>>>
>>> I'm not sure assuming linear (or actually any) ordering between
>>> variants is a good thing. Yes, right now we only have gnttab
>>> v1 and v2 and evtchn 2l and fifo, which could be considered v1
>>> and v2 as you suggest. However, assuming a 3rd variant surfaces,
>>> why would it be that one has to expose v2 just to make v3
>>> usable? In particular gnttab v2 has various issues (which is why
>>> you introduced a way to disable its use in the first place), yet
>>> I'd hope we'd end up with a less quirky v3 if one ever becomes
>>> necessary. And in turn I'd hope we could hide v2 from any v3
>>> users.
>>>
>>> IOW I think a bitmap to permit use of "advanced" versions is
>>> more future proof. (As a side note, I don't think we want to
>>> introduce a disable for the respective v1 interfaces.)
>>
>> We absolutely do want a way to turn everything off.
>>
>> The inability to turn the Xen extensions off for HVM guests (even if
>> only for debugging purposes) is a severely short sighted decision.
>
> For HVM perhaps, but not for PV.
Right...
I'm confused as to what in my sentence is in any way unclear.
>
>> It is also a feature which has been requested repeatedly by users in the
>> past, and I am very deliberately building a way to do this into the
>> CPUID work.
>>
>> However, it is an unreasonable request to bundle into this bugfix, hence
>> why I didn't suggest it.
>
> There's no bug fix here, as there's no bug (in Xen).
? I didn't say it was a bug in Xen, but the change is specifically to
fix a bug.
>
>> Now I think about it, things like available ABIs really should be in the
>> Xen hypervisor CPUID leaves, but again, that ship sailed a decade ago.
>
> For comparison, do you know of any CPU architecture making _all_
> of its basic insns and other functionality available conditionally
> only?
No, but that's also not what I'm suggesting.
For an HVM guest, literally everything Xen-specific, from hypercalls to
CPUID leaves to xenstored and PV drivers, is an extension on a base
IBM-compatible PC.
There should always have been a way of of running an HVM guest without
any of this, and we (the Xen community) really do have users which want
to be able to do this.
~Andrew
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next prev parent reply other threads:[~2019-08-07 15:57 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-08-07 11:20 [Xen-devel] [PATCH] evtchn: make support for different ABIs tunable Eslam Elnikety
2019-08-07 11:40 ` Andrew Cooper
2019-08-07 12:04 ` Woodhouse, David
2019-08-07 12:18 ` Jan Beulich
2019-08-07 12:24 ` Andrew Cooper
2019-08-07 12:07 ` Elnikety, Eslam
2019-08-07 12:20 ` Jan Beulich
2019-08-07 13:27 ` Elnikety, Eslam
2019-08-07 12:30 ` Andrew Cooper
2019-08-07 13:01 ` Elnikety, Eslam
2019-08-07 13:41 ` Andrew Cooper
2019-08-07 14:30 ` Jan Beulich
2019-08-07 15:00 ` Andrew Cooper
2019-08-07 15:08 ` Jan Beulich
2019-08-07 15:57 ` Andrew Cooper [this message]
2019-08-07 16:03 ` Jan Beulich
2019-08-14 12:51 ` George Dunlap
2019-08-14 13:02 ` Andrew Cooper
2019-08-19 12:16 ` Eslam Elnikety
2019-08-07 15:43 ` Elnikety, Eslam
2019-08-07 14:35 ` Jan Beulich
2019-08-07 14:41 ` Julien Grall
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