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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 30.04.14 at 02:38, <aravindp@xxxxxxxxx> wrote:
> 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.

Jan


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