Re: [Xen-devel] [RFC] DOMCTL_memattrs_op : a new DOMCTL to play with stage-2 page attributes

[George Dunlap <george.dunlap@xxxxxxxxxx>]

On 07/07/2017 08:27 AM, Jan Beulich wrote:
>>>> On 06.07.17 at 20:07, <julien.grall@xxxxxxx> wrote:
>> On 07/06/2017 06:42 PM, Stefano Stabellini wrote:
>>> On Thu, 6 Jul 2017, Jan Beulich wrote:
>>>> Considering the intended purpose here (as far as I recall it), was it
>>>> already taken into consideration to request suitable attributes right
>>>> at the time the page gets installed into the physmap? Iirc there's no
>>>> need to actually "play" with the attributes at random times.
>>>
>>> This operation would be done before the guest starts.
>>>
>>>
>>> Let's give a look at the list the changes that would be required to make
>>> these hypercalls suitable for this task:
>>>
>>> 1) remove the dependency on CONFIG_HAS_MEM_ACCESS
>>> 2) remove the p2m_mem_access_sanity_check check for these two hypercalls
>>> 3) remove the (!d->vm_event->monitor.ring_page) check for these two 
>>> hypercalls
>>> 4) prevent p2m->mem_access_enabled from being set for these two hypercalls
>>>
>>> Am I missing anything? After we do this, would they still be useful for
>>> their original mem_access related purpose?
>>
>> But how would you handle mem_access on those regions in that case? This 
>> looks completely incompatible.
>>
>> The memaccess code has to store the previous permission in order to look 
>> for the fault. Here you want to modify for good.
>>
>> Furthermore, memaccess is only here to modify permission. It does not 
>> handle cacheability... So it looks to me you are trying to re-purposing 
>> an hypercall that will not fit all our needs in the future.
>>
>> I think the way forward is to introduce an hypercall which populate/map 
>> memory with a given set of attributes and permissions.
>>
>> This would simplify quite a lot the logic (one hypercall instead of 
>> multiple one) and avoid to worry about attributes changed multiple time 
>> even before the guest is booting.
> 
> Right - that's what I was suggesting with the last paragraph of my
> previous reply; I have to admit that I have trouble seeing how
> Stefano's response relates to that.

I had a hard time interpreting that paragraph.  Did you mean:

"Have you considered trying to populate the p2m table with the correct
permissions when first populating it, rather than populating it with
plain rw ram and then changing it afterwards?"

 -George

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
https://lists.xen.org/xen-devel