Re: [Xen-devel] EPT question - XENMEM_get_access_op

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 20.02.15 at 12:44, <bsingharora@xxxxxxxxx> wrote:
> On Fri, Feb 20, 2015 at 4:19 PM, Jan Beulich <JBeulich@xxxxxxxx> wrote:
>>>>> On 20.02.15 at 05:47, <bsingharora@xxxxxxxxx> wrote:
>>> I'm trying to use the memevent channel to extract the permission for a
>>> particular text section. I expect to see rx as the permission, instead
>>> I am seeing rwx. I have not spent a whole lot of time looking at
>>> memaccess[] and p2m-ept.c, but I am puzzled. The OS definitely has
>>> this text segment marked as COPY-ON-WRITE, I am a little confused with
>>> the permissions returned. Is there a way to get the right thing?
>>
>> What the guest OS specifies doesn't matter here - EPT is not OS
>> (L1) but hypervisor (L2) paging, and hence the permissions there
>> are controlled by the hypervisor. Of course guest memory
>> accesses have to pass both paging levels' access checks in order
>> to be carried out (and not fault).
> 
> 
> Thanks Jan! Is there a way for a memevents channel consumer to get
> access to the L1 (OS Page tables).

Hardly.

> I presume we'll need to walk the
> page tables, I suspect the current access_op is broken without it and
> may not be returning anything meaningful in most of the cases

It's not broken in any way, you just seem to have wrong expectations.

Jan


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