Re: unhandled word causes Xen crash with recent Linux kernels, was: Re: [PATCH v2 05/11] xen/arm: vgic: Properly emulate the full register

[Ian Campbell <ian.campbell@citrix.com>]

On Mon, 2015-11-30 at 12:22 +0000, Julien Grall wrote:
> Hi Ian,
> 
> On 25/11/15 12:26, Ian Campbell wrote:
> > On Wed, 2015-11-25 at 12:15 +0000, Stefano Stabellini wrote:
> > > On Wed, 25 Nov 2015, Shannon Zhao wrote:
> > > > Upstream Linux kernel applies below patch which will write
> > > > GICD_ICACTIVER. But since Xen doesn't support it, so it will cause
> > > > Dom0
> > > > initializes GIC failed.
> > > > 
> > > > 0eece2b22849c90b730815c893425a36b9d10fd5 (irqchip/gic: Make sure
> > > > all
> > > > interrupts are deactivated at boot)
> > > > 
> > > > (XEN) d0v0: vGICD: unhandled word write 0xffffffff to ICACTIVER4
> > > > (XEN) traps.c:2447:d0v0 HSR=0x93860046 pc=0xffffffc0008d63f0
> > > > gva=0xffffff8000004384 gpa=0x0000002f000384
> > > > (XEN) DOM0: Unhandled fault: ttbr address size fault (0x96000000)
> > > > at
> > > > 0xffffff8000004384
> > > > (XEN) DOM0: Internal error: : 96000000 [#1] PREEMPT SMP
> > > > (XEN) DOM0: Modules linked in:
> > > > (XEN) DOM0: CPU: 0 PID: 0 Comm: swapper/0 Not tainted 4.4.0-rc2+
> > > > #364
> > > > (XEN) DOM0: Hardware name: (null) (DT)
> > > > (XEN) DOM0: task: ffffffc000969970 ti: ffffffc00095c000 task.ti:
> > > > ffffffc00095c000
> > > > (XEN) DOM0: PC is at gic_dist_config+0x78/0xa0
> > > > (XEN) DOM0: LR is at __gic_init_bases+0x240/0x2bc
> > > > 
> > > > Do we have a plan to fix this?
> > > 
> > > Thanks for the reporting the issue, I can reproduce the
> > > problem.  Given
> > > that this is a very serious regression and that we cannot really
> > > "fix"
> > > the Linux side because Linux is not doing anything wrong, I think we
> > > have to go with a very simple change, something we can easily
> > > backport
> > > to all past Xen releases.
> > > 
> > > I suggest we turn the "unhandled word write" into a write_ignore, see
> > > below:
> > 
> > As discussed IRL this might be tolerable as a patch intended for
> > backporting purposes, but I would want to see it in a series along with
> > one
> > or more not-for-backport patches which actually makes the register work
> > as
> > it should.
> 
> I have the feeling that fixing properly GICD_I*ACTIVER will take
> sometimes as we also need to take into consideration hardware interrupt
> routed to a guest.
> 
> As this is preventing Linux upstream to run on the latest, can we get a
> simple fix for now?

With a suitable commit message explaining the interim/backportability
nature of this patch and the intention to do it properly I'd be willing to
accept it.

Ian.

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