Re: [Xen-devel] Issue policing writes from Xen to PV domain memory

["Aravindh Puthiyaparambil (aravindp)" <aravindp@xxxxxxxxx>]

>>>> > On adding some debugging, I discovered that it happens after
>>>> > mem_access
>>>>is
>>>>> enabled but xen-access has not started handling events. After
>>>>> comparing
>>>>the
>>>>> stack trace and gla in question There are multiple write faults to
>>>>> the runstate_guest(v), each causing an event to be sent to
>>>>> xen-access. Since
>>>>the
>>>>> listener is not handling events yet, the fault continues to occur. I
>>>>> am not sure why the listener does not get a chance to run.  I also
>>>>> do not follow is that why there are multiple faults as the vcpu
>>>>> should have been paused
>>>>after
>>>>> the first event was sent to xen-access and only be resumed after
>>>>> violation
>>>>has
>>>>> been resolved and when it calls xc_access_resume(), which ends up
>>>>unpausing
>>>>> the vcpu. Or is this occurring because runstate_guest(v).p is being
>>>>> accessed from Xen?
>>>>
>>>>The runstate changes (and hence needs to get written) as a side effect
>>>>of pausing the guest (as can be seen from the stack trace). The first
>>>>question that needs clarification (for me at least, since I don't know
>>>>much about the access stuff for HVM) is how the same situation gets
>>>>handled
>>>>there: Do Xen writes to HVM guest memory get intercepted? Other than
>>>>for PV, they're not going through the same page tables, so special
>>>>precautions would be needed to filter them. Quite obviously (I think)
>>>>if they're not being filtered for HVM, then they shouldn't be for PV.
>>>
>>> AFAIK, they are not being filtered for HVM. I brought up a HVM domain
>>> and printed out the runstate_guest area value when it is registered. I
>>> then ran the xen-access test program to monitor for writes.
>>> Interestingly I never saw a GLA that matched the runstate_guest area.
>>> This is not the case for a PV domain as it is one of the first violations 
>>> the
>xen-
>>access test program sees.
>>> Is this an EPT vs regular page table difference as in one case it is shared?
>>
>>Yes, as said in my earlier reply. And when hypervisor writes aren't subject to
>>filtering in HVM, you probably want/need to make things behave that way
>for
>>PV too (albeit it may not be trivial, as you clearly don't want to start
>emulating
>>hypervisor accesses).
>
>The main issue I am seeing is the same violation being sent multiple times to
>the mem_event queue and the listener not getting scheduled to run. I would
>think that the same issue would happen with a HVM guest but in that case I
>don't even see a violation of the runstate_guest area. I guess I should try
>figuring out why wqv->esp becomes 0 instead of trying to stop multiple
>events for the same violations being sent. If you have a suggestion in the area
>I should be looking at, it would be welcome.

I meant to say why wqv->esp becomes non-zero in the message above.

Thanks,
Aravindh

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