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

["Woodhouse, David" <dwmw@xxxxxxxxxxxx>]

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.

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.

I wonder if treating it more like a kexec and doing SHUTDOWN_soft_reset
in the handover might be a viable long-term approach (and deal with
other KASLR-related problems). Not that soft reset currently resets
this AFAICT at a quick glance, but maybe it should? In the meantime
though, hiding 2L mode for guests which were first booted without it is
a simple option which doesn't force guests to upgrade.

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