Re: [Xen-devel] Altp2m use with PML can deadlock Xen

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 10/05/2019 16:09, Tamas K Lengyel wrote:
> On Fri, May 10, 2019 at 8:59 AM Andrew Cooper <andrew.cooper3@xxxxxxxxxx> 
> wrote:
>> On 10/05/2019 15:53, Razvan Cojocaru wrote:
>>> On 5/10/19 5:42 PM, Tamas K Lengyel wrote:
>>>> On Thu, May 9, 2019 at 10:19 AM Andrew Cooper
>>>> <andrew.cooper3@xxxxxxxxxx> wrote:
>>>>> On 09/05/2019 14:38, Tamas K Lengyel wrote:
>>>>>> Hi all,
>>>>>> I'm investigating an issue with altp2m that can easily be reproduced
>>>>>> and leads to a hypervisor deadlock when PML is available in hardware.
>>>>>> I haven't been able to trace down where the actual deadlock occurs.
>>>>>>
>>>>>> The problem seem to stem from hvm/vmx/vmcs.c:vmx_vcpu_flush_pml_buffer
>>>>>> that calls p2m_change_type_one on all gfns that were recorded the PML
>>>>>> buffer. The problem occurs when the PML buffer full vmexit happens
>>>>>> while the active p2m is an altp2m. Switching  p2m_change_type_one to
>>>>>> work with the altp2m instead of the hostp2m however results in EPT
>>>>>> misconfiguration crashes.
>>>>>>
>>>>>> Adding to the issue is that it seem to only occur when the altp2m has
>>>>>> remapped GFNs. Since PML records entries based on GFN leads me to
>>>>>> question whether it is safe at all to use PML when altp2m is used with
>>>>>> GFN remapping. However, AFAICT the GFNs in the PML buffer are not the
>>>>>> remapped GFNs and my understanding is that it should be safe as long
>>>>>> as the GFNs being tracked by PML are never the remapped GFNs.
>>>>>>
>>>>>> Booting Xen with ept=pml=0 resolves the issue.
>>>>>>
>>>>>> If anyone has any insight into what might be happening, please let
>>>>>> me know.
>>>>>
>>>>> I could have sworn that George spotted a problem here and fixed it.  I
>>>>> shouldn't be surprised if we have more.
>>>>>
>>>>> The problem that PML introduced (and this is mostly my fault, as I
>>>>> suggested the buggy solution) is that the vmexit handler from one vcpu
>>>>> pauses others to drain the PML queue into the dirty bitmap.  Overall I
>>>>> wasn't happy with the design and I've got some ideas to improve it, but
>>>>> within the scope of how altp2m was engineered, I proposed
>>>>> domain_pause_except_self().
>>>>>
>>>>> As it turns out, that is vulnerable to deadlocks when you get two vcpus
>>>>> trying to pause each other and waiting for each other to become
>>>>> de-scheduled.
>>>> Makes sense.
>>>>
>>>>> I see this has been reused by the altp2m code, but it *should* be safe
>>>>> to deadlocks now that it takes the hypercall_deadlock_mutext.
>>>> Is that already in staging or your x86-next branch? I would like to
>>>> verify that the problem is still present or not with that change. I
>>>> tested with Xen 4.12 release and that definitely still deadlocks.
>>> I don't know if Andrew is talking about this patch (probably not, but
>>> it looks at least related):
>>>
>>> http://xenbits.xen.org/gitweb/?p=xen.git;a=commitdiff;h=24d5282527f4647907b3572820b5335c15cd0356;hp=29d28b29190ba09d53ae7e475108def84e16e363
>>>
>> I was referring to 29d28b2919 which is also in 4.12 as it turns out.
>> That said, 24d5282527 might in practice be the cause of the deadlock, so
>> I'd first experiment with taking that fix out.
>>
>> I know for certain that it won't be tested with PML enabled, because the
>> use of PML is incompatible with write-protecting guest pagetables.
>>
> Sounds like it's the safe bet to just have PML be disabled for when
> introspection is used. I would say it would be even better if the use
> of PML could be controlled on a per-guest base instead of the current
> global on/off switch. That way it could be disabled only for the
> introspected domains.
>
> I'll do some more experimentation when I get some free time but two
> observations that speak against the vCPUs trying to pause each other
> being the culprit is that:
> - the deadlock doesn't happen with xen-access' altp2m use, it only
> happens when there are remapped gfn's in the altp2m views
> - I've added a domain_pause/unpause to the PML flusher before it
> enters the flush loop but I still got a deadlock

Do you have a minimal repro of the deadlock you could share?

Even if it is a combo PML+altp2m problem, we should fix the issue,
because there are VMI usecases which don't care about write-protecting
guest pagetables, and we don't want to prevent those cases from using PML.

Thanks,

~Andrew

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