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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 07.07.17 at 10:56, <george.dunlap@xxxxxxxxxx> wrote:
> 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?"

Yes. I'm sorry for having badly expressed myself.

Jan


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