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?

Thanks,
Aravindh


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