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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 07.08.2019 14:04,  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.
>
> 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?
Oh, definitely, if it doesn't already. Soft-reset should really do what
its name says and put state back to what it was initially.

Jan

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