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Re: [Xen-devel] [PATCH] evtchn: make support for different ABIs tunable
- To: "Woodhouse, David" <dwmw@xxxxxxxxxxxx>, "Elnikety, Eslam" <elnikety@xxxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Wed, 7 Aug 2019 13:24:17 +0100
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- Delivery-date: Wed, 07 Aug 2019 12:24:30 +0000
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- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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On 07/08/2019 13:04, Woodhouse, David wrote:
> On Wed, 2019-08-07 at 12:40 +0100, Andrew Cooper 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.
> On resume, the guest first boots a Linux kernel (the 'boot' kernel).
> That kernel then loads the previous state (the 'resumed' kernel) from
> disk and then transfers control to it.
Right, so the 'boot' kernel is setting up a mode, which the on-disk
kernel/state can't cope with.
> I believe the problem occurs when the boot kernel sees and enables FIFO
> mode, then transfers control to the resumed kernel which is expecting
> 2L.
And I presume the underlying mess here is because virtual subsystems
aren't saved/restored like most other devices?
Ultimately, this looks like a Linux bug, but in the principle of "the
virtual environment a guest sees should be constant for its logical
lifetime", restricting the visible ABI probably is right thing to do.
~Andrew
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