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

["Elnikety, Eslam" <elnikety@xxxxxxxxxx>]

> 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.
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