Re: [Xen-devel] [v5][PATCH 03/10] xen:x86: define a new hypercall to get RMRR mappings

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 03.09.14 at 10:31, <tiejun.chen@xxxxxxxxx> wrote:
> On 2014/9/3 9:45, Chen, Tiejun wrote:
>> On 2014/9/2 21:15, Jan Beulich wrote:
>>>>>> On 02.09.14 at 13:10, <tiejun.chen@xxxxxxxxx> wrote:
>>>> On 2014/9/2 18:15, Jan Beulich wrote:
>>>>>>>> On 02.09.14 at 11:59, <tiejun.chen@xxxxxxxxx> wrote:
>>>>>> +    case XENMEM_reserved_device_memory_map:
>>>>>> +    {
>>>>>> +        struct xen_mem_reserved_device_memory *xmrdm = NULL;
>>>>>> +        struct xen_mem_reserved_device_memory_map xmrdmm;
>>>>>> +        XEN_GUEST_HANDLE(xen_mem_reserved_device_memory_t) buffer;
>>>>>> +        XEN_GUEST_HANDLE_PARAM(xen_mem_reserved_device_memory_t)
>>>>>> buffer_param;
>>>>>> +        const struct iommu_ops *ops = iommu_get_ops();
>>>>>> +        unsigned int nr_entries = 0;
>>>>>> +        unsigned int i = 0;
>>>>>> +
>>>>>> +        xmrdm = ops->get_device_reserved_memory(&nr_entries);
>>>>
>>>> Do we still need this iommu_ops somewhere else?
>>>
>>> Not this one, but another one (as I had described further down).
>>>
>>>>>> +        if ( !nr_entries )
>>>>
>>>> Do we still need this 'nr_entries' here?
>>>
>>> Doesn't look like so. But it's you coding this up - you ought to clearly
>>> see what is needed and what not.
>>>
>>
>> I mean we need to get 'nr_entries' before any real operations since if
>> that is zero, any following operations are pointless. And even that is
>> nonzero, but actually the user don't know how many buffer should be
>> passed, so often we may need to notify user again at the first time then
>> the user call this again with a appropriate buffer. Right?
>>
>> So how do we get this 'nr_entries'? Seems we have to to go VT-D stuff to
>> walk that list or other similar approaches. If so, why we still get
>> those real mapping entries later by accessing VT-D stuff again?
>>
>> Shouldn't we figure out a approach we just call one time?
>>
> 
> I update this patch based on this point:

I'm afraid I have to give up and instead go and implement this for
you (which already by now would clearly have been the much less
time consuming thing at least on my end).

Jan


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