Re: [Xen-devel] [PATCH] x86/altp2m: Add a subop for obtaining the mem access of a page

[George Dunlap <George.Dunlap@xxxxxxxxxx>]

> On Jul 2, 2018, at 7:34 AM, Jan Beulich <JBeulich@xxxxxxxx> wrote:
> 
>>>> On 29.06.18 at 18:39, <rcojocaru@xxxxxxxxxxxxxxx> wrote:
>> On 06/29/2018 06:38 PM, Jan Beulich wrote:
>>>>>> On 28.06.18 at 15:00, <apop@xxxxxxxxxxxxxxx> wrote:
>>>> @@ -4666,6 +4667,23 @@ static int do_altp2m_op(
>>>>         }
>>>>         break;
>>>> 
>>>> +    case HVMOP_altp2m_get_mem_access:
>>>> +        if ( a.u.mem_access.pad )
>>>> +            rc = -EINVAL;
>>>> +        else
>>>> +        {
>>>> +            xenmem_access_t access;
>>>> +
>>>> +            rc = p2m_get_mem_access(d, _gfn(a.u.mem_access.gfn), &access,
>>>> +                                    a.u.mem_access.view);
>>>> +            if ( !rc )
>>>> +            {
>>>> +                a.u.mem_access.hvmmem_access = access;
>>>> +                rc = __copy_to_guest(arg, &a, 1) ? -EFAULT : 0;
>>> 
>>> __copy_field_to_guest()? Or wait, no, the function argument is still a
>>> handle of void.
>>> 
>>> And then - here we are again: Is it reasonable to permit a domain inquiring
>>> for itself?
>> 
>> A good question. Perhaps the following are decision factors:
>> 
>> 1. It is already possible for a domain to set mem_access restrictions
>> (via HVMOP_altp2m_set_mem_access) on itself.
> 
> Which, as before, I consider a flaw.

How many times do we have to go over this?  Here is my recollection from the 
last time we had a discussion on this topic:

* The original authors of this code probably thought having guests set their 
own memaccess would be a potential use case
* The maintainers and main users of the code (Tamas and Razvan) think it’s a 
useful use case
* The MM maintainer (me) and one of the x86 maintainers (Andy) think it’s a 
useful use case.

(Correct me if I’ve misremembered anywhere.)

Do we need to have a formal vote by the committers for you to accept that this 
should be a supported use case, and stop making objections any time someone 
wants to improve it?

As long as not being able to call altp2m_set_mem_access  means not being able 
to call altp2m_get_mem_access, this should be fine.

 -George
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