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

[Julien Grall <julien.grall@arm.com>]

Hi,

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
>>>> injecting
>>>> 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.
>>>>
>>>> Cheers,
>>>>
>>>> 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
>>>
>>> Thanks.
>>>
>>> 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
>> them.
>>
>> 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 
enable?

Stefano, what's your opinion here?

Cheers,

-- 
Julien Grall

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