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I've also tested the patch with LibVMI and everything works fine. The pause/unpause reference count now does take effect, so the previous issue I reported (a paused domain getting unpaused by mem_event_enable) is fixed by this patch.

One question I have, what if the toolstack wants to unconditionally (force) unpause a domain? Right now with this patch if someone runs 'xl pause domain' a couple times he has no other recourse then to issue 'xl unpause domain' at least the same number of times, or to restart the entire domain. Might be user-friendlier if there was an override provided in case a domain got paused a million times by accident.

Cheers,
Tamas

On Thu, Jul 17, 2014 at 10:26 PM, Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx> wrote:

On 07/17/2014 10:01 PM, Aravindh Puthiyaparambil (aravindp) wrote:
>> Xen performs insufficient validation of the contents of mem_event responses
>>from the toolstack. As a result, a buggy toolstack could cause Xen to walk off
>> the end of a domain's vcpu list, and get out of sync with vcpu pause reference
>> counts.
>>
>> These two fixes are compile tested only, as I have no way to plausibly test the
>> mem-event functionality itself.
>
> One easy way of testing is to use the tools/tests/xen-access test program which exercises mem_access and thereby mem_event. It is fairly easy to run. Bring up a domain and execute " xen-access <domain_id> write|exec". But I understand if you are under time constraints and cannot do it. If you Cc me on these patches, I will gladly test them for you.

Indeed, our application is very xen-access-like (except quite a bit more
involved), and I've tested the original patches with 5 different domains
3 times over - but it's a well-behaved citizen of the Xen ecosystem and
there were no gimmicks involved. No mem_events piled up, and there was
always just one mem_event handler per domain.

Everything went without a hitch, but I did not try to pause the domain
while it was running or try to trick the hypervisor in any way.

Thanks,
Razvan Cojocaru

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Re: [Xen-devel] [PATCH 0/2] Xen/mem_event: Do not rely on the toolstack being bug-free