Re: [Xen-devel] [PATCH 00/11] Alternate p2m: support multiple copies of host p2m

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 14.01.15 at 11:31, <tamas.lengyel@xxxxxxxxxxxx> wrote:
> On Wed, Jan 14, 2015 at 8:04 AM, Jan Beulich <jbeulich@xxxxxxxx> wrote:
>>>>> Ed White <edmund.h.white@xxxxxxxxx> 01/13/15 10:32 PM >>>
>>>On 01/13/2015 12:45 PM, Andrew Cooper wrote:
>>>> On 13/01/15 20:02, Ed White wrote:
>>>>> The set of mfn's is the same, but I do allow gfn->mfn mappings to be
>>>>> modified under certain circumstances. One use of this is to point the
>>>>> same VA to different physical pages (with different access permissions)
>>>>> in different p2m's to hide memory changes.
>>>>
>>>> What is the practical use of being able to play paging tricks like this
>>>> behind a VMs back?
>>>
>>>I'm restricted in how much detail I can go into on a public mailing list,
>>>but imagine that you want a data read to see one thing and an instruction
>>>fetch to see something else.
>>
>> How would that work? There can only be one P2M in use at a time, and that's
>> used for both translations. Or are you saying at least one of the two 
>> accesses
>> would be emulated nevertheless?
> 
> I can see it working by having data fetch access to a page trapped via
> mem_access, while instruction fetch is not.

Understood, but how do you then carry out the data access? The
question I raised was whether that would then involve emulation.

Jan


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