Re: [Xen-devel] [PATCH] evtchn: make support for different ABIs tunable

From: Andrew Cooper <andrew.cooper3@citrix.com>
To: "Elnikety, Eslam" <elnikety@amazon.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>,
	Julien Grall <julien.grall@arm.com>,
	Jan Beulich <jbeulich@suse.com>,
	Anthony PERARD <anthony.perard@citrix.com>,
	"xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>,
	"Woodhouse, David" <dwmw@amazon.co.uk>
Subject: Re: [Xen-devel] [PATCH] evtchn: make support for different ABIs tunable
Date: Wed, 7 Aug 2019 13:30:38 +0100	[thread overview]
Message-ID: <49a95cf4-0b76-234b-ebef-2ed290ef5330@citrix.com> (raw)
In-Reply-To: <9F89B8B5-D92E-4CA6-ABCF-5F8E9707AE94@amazon.com>

On 07/08/2019 13:07, Elnikety, Eslam wrote:
>> On 7. Aug 2019, at 13:40, Andrew Cooper <andrew.cooper3@citrix.com> 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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