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Re: [Xen-devel] [PATCH] evtchn: make support for different ABIs tunable
- To: "Elnikety, Eslam" <elnikety@xxxxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Wed, 7 Aug 2019 13:30:38 +0100
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- Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>, George Dunlap <George.Dunlap@xxxxxxxxxxxxx>, Tim Deegan <tim@xxxxxxx>, Ian Jackson <ian.jackson@xxxxxxxxxxxxx>, Julien Grall <julien.grall@xxxxxxx>, Jan Beulich <jbeulich@xxxxxxxx>, Anthony PERARD <anthony.perard@xxxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>, "Woodhouse, David" <dwmw@xxxxxxxxxxxx>
- Delivery-date: Wed, 07 Aug 2019 12:30:54 +0000
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- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
- Openpgp: preference=signencrypt
On 07/08/2019 13:07, Elnikety, Eslam wrote:
>> On 7. Aug 2019, at 13:40, Andrew Cooper <andrew.cooper3@xxxxxxxxxx> wrote:
>>
>> On 07/08/2019 12:20, Eslam Elnikety wrote:
>>> Adding support for FIFO event channel ABI was first introduced in Xen 4.4
>>> (see 88910061ec6). Make this support tunable, since the choice of which
>>> event channel ABI has implications for hibernation. Consider resuming a
>>> pre Xen 4.4 hibernated Linux guest. The guest boot kernel defaults to FIFO
>>> ABI, whereas the resume kernel assumes 2L. This, in turn, results in Xen
>>> and the resumed kernel talking past each other (due to different protocols
>>> FIFO vs 2L).
>> I'm afraid I don't follow.
>>
>> We have a Linux kernel which knows about FIFO, which was first booted on
>> Xen < 4.4, so configured 2L mode.
>>
>> It is then suspended, and resumed on a newer Xen >= 4.4. The guest now
>> has a choice between 2L mode, and FIFO mode.
>>
>> What is the problem?
>>
>> When resuming, the guest in question should continue to use 2L mode,
>> because that is what it was using previously.
>>
>> ~Andrew
>
> After resuming (i.e., Linux's software_resume), the guest will indeed
> continue to use 2L. However, Xen has already done evtchn_fifo_init_control as
> part of the boot kernel init (before the guest's software_resume). Then, we
> reach the point where guest assumes 2L and Xen assumes FIFO.
With Davids explanation, I now understand the problem. However for
clarity, it is probably worth making a correction here.
It isn't Xen which does evtchn_fifo_init_control(). It is the "boot"
kernel issuing EVTCHNOP_init_control hypercall which switches the domain
from 2L mode into FIFO mode.
Xen is doing exactly as it was instructed. The underlying bug is a
mismatch in behaviour between the "boot" kernel and the on-disk
kernel/state.
~Andrew
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