Re: [Xen-devel] [PATCH v2 00/12] Alternate p2m: support multiple copies of host p2m

[Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>]

On 06/24/2015 12:27 AM, Lengyel, Tamas wrote:
> I've extended xen-access to exercise this new feature taking into
> account some of the current limitations. Using the altp2m_write|exec
> options we create a duplicate view of the default hostp2m, and instead
> of relaxing the mem_access permissions when we encounter a violation, we
> swap the view on the violating vCPU while also enabling MTF
> singlestepping. When the singlestep event fires, we use the response to
> that event to swap the view back to the restricted altp2m view.

That's certainly very interesting. I wonder what the benefits are in
this case over emulating the fault-causing instruction (other than
obviously not going through the emulator)? The altp2m method would
certainly be slower, since you need more round-trips from userspace to
the hypervisor (the EPT vm_event handling + the singlestep event,
whereas with emulation you just reply to the original vm_event).


Regards,
Razvan

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