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Re: [Xen-devel] [PATCH RFC V9 4/5] xen, libxc: Request page fault injection via libxc

On Tue, Sep 2, 2014 at 2:24 PM, Tim Deegan <tim@xxxxxxx> wrote:
> Hi,
> At 12:18 +0300 on 02 Sep (1409656686), Razvan Cojocaru wrote:
>> While we need to set the data per-domain and have whatever VCPU inject
>> the page fault - _but_only_if_ it's in usermode and its CR3 points to
>> something interesting.
> That's a strange and specific thing to ask the hypervisor to do for
> you.  Given that you can already trap CR3 changes as mem-events can
> you trigger the fault injection in response to the contect switch?
> I guess that would probably catch it in kernel mode. :(

I was wondering, rather than special-casing inject_trap, would it make
sense to be able for the memory controller to get notifications when
certain more complex conditions happen (e.g., "some vcpu is in user
mode with this CR3")?  Then the controller could ask to be notified
when the event happens, and when it does, just call inject_fault.

That way, inject_fault isn't special-cased at all; and one could
imagine designing the "condition" such that any number of interesting
conditions could be trapped.


But ultimately, as Tim said, you're basically just *hoping* that it
won't take too long to happen to be at the hypervisor when the proper
condition happens.  If the process in question isn't getting many
interrupts, or is spending the vast majority of its time in the
kernel, you may end up waiting an unbounded amount of time to be able
to "catch" it in user mode.  It seems like it would be better to find
a reliable way to trap on the return into user mode, in which case you
wouldn't need to have a special "wait for this complicated event to
happen" call at all, would you?


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