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

[Ed White <edmund.h.white@xxxxxxxxx>]

On 06/24/2015 06:37 AM, Razvan Cojocaru wrote:
> On 06/24/2015 04:32 PM, Lengyel, Tamas wrote:
>>
>>
>> On Wed, Jun 24, 2015 at 1:39 AM, Razvan Cojocaru
>> <rcojocaru@xxxxxxxxxxxxxxx <mailto:rcojocaru@xxxxxxxxxxxxxxx>> wrote:
>>
>>     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
>>
>>
>> Certainly, this is pretty slow right now, especially for the altp2m_exec
>> case. However, sometimes you simply cannot emulate. For example if you
>> write breakpoints into target locations, the original instruction has
>> been overwritten with 0xCC. If you have a duplicate of the page without
>> the breakpoint, this is an easy way to make the guest fetch the original
>> instruction. Of course, if you extend the emulation routine where you
>> can provide the instruction to emulate, instead of it being fetched from
>> guest memory, that would be equally useful ;)
> 
> Makes sense, thanks for the explanation! Sure, sending back the
> instruction to emulate could be something to consider for the future.
> 
> 
> Thanks,
> Razvan
> 

One thing I'd add is that what Tamas has done provides a valuable test that
the cross-domain functionality works, even if it might not be a recommended
design pattern. Our primary use case at Intel is intra-domain, and there
the advantages of avoiding many exits are clear.

Also, even cross-domain usage allows for different views of, and levels of
access to, memory concurrently on different vcpus.

Ed


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