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Re: [Xen-devel] [PATCH 0/3] xen/arm: Inject an exception to the guest rather than crashing it

Hi Stefano,

On 30/01/18 19:21, Stefano Stabellini wrote:
On Tue, 30 Jan 2018, Julien Grall wrote:

On 30/01/18 18:29, Andrew Cooper wrote:
On 30/01/18 17:00, Julien Grall wrote:

On 30/01/18 16:38, Andrew Cooper wrote:
On 30/01/18 16:14, Julien Grall wrote:
Hi all,

This small series replaces all call to domain_crash_synchronous by
an exception to the guest.

This will result to a nicer trace from the guest (no need to
manually walk
the stack) and give a chance to the guest to give a bit more
information on
what it was doing.


Julien Grall (3):
     xen/arm: io: Distinguish unhandled IO from aborted one
     xen/arm: Don't crash domain on bad MMIO emulation
     xen/arm: Don't crash the domain on invalid HVC immediate


I don't feel qualified to review these, but some notes.

Patch 1.  s/avodi/avoid/ in the commit message

Patches 2 and 3.  You probably want to convert the printks to
gdprintk()s, otherwise guests can choke up the ratelimited log.  Doing
so will also mean that the vcpu will be identified consistently, which
it isn't currently.
We didn't use g*printk because it would be more confusing to print the
current vCPU in some cases (e.g when accessing the re-distributor of
another vCPU) or does not matter (e.g for ITS).

In the former case, you'd want to print both current, and the target
vcpu.  The latter still matters what current is if something goes wrong.

We have plenty of similar cases in x86, but at the point you are
printing an diagnostic message, ignoring current is almost always the
wrong think to do.

I will look at it on another series.

The problem with the debug version is those information are actually
quite useful in non-debug build. We found quite a few issues thanks to

I think it would make more sense for Xen to provide per-guest
ratelimited than hiding those messages in non-debug build.

Per guest is quite a lot more complicated than global, and would still
require a global limit to prevent a concerted attack from multiple
guests to avoid DoSing the system.

Debug vs unilateral is your prerogative as a maintainer, but as you've
said yourself, the are used for debugging purposes, which proves my point.

So on x86, you always request the user to reproduce it with debug build

Stefano, what's your opinion here?

I think it would be great to have per-guest rate limiting.

On ARM, it would be impossible to ask users to repro with debug enabled,
given that many deployments are embedded (set-top boxes, etc).

I think it is OK to keep the XENLOG_DEBUG, they should be filtered out
by loglvl. But we need to be careful about the others. We might want to
convert the XENLOG_WARNING in traps.c to XENLOG_DEBUG: they are warning
about guests misbehaving, but from the hypervisor point of view, it's not
a problem, guests can shoot themselves if they want to; it's OK.

Not really. Some of the XENLOG_WARNING may happen because Xen does not handle a trap (see the one on do_trap_guest_sync). That has nothing to do with a guest misbehaving and could happen if Xen boots on newer hardware.


Julien Grall

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