Re: [PATCH] xen: add privcmd ioctl to get p2pdma_distance

[Jan Beulich <jbeulich@xxxxxxxx>]

On 17.12.2024 06:53, Zhang, Julia wrote:
> 
> 
> On 2024/12/16 17:19, Jan Beulich wrote:
>> On 16.12.2024 09:18, Zhang, Julia wrote:
>>> On 2024/12/9 15:47, Jan Beulich wrote:
>>> On 07.12.2024 11:59, Julia Zhang wrote:
>>
>> Yet another formality, sorry: Please send plain text emails. You'll note 
>> that what
>> I said and why you said is indistinguishably intermixed below.
> 
> Thanks for reminding.
> 
>>
>>> --- a/tools/include/xen-sys/Linux/privcmd.h
>>>
>>> +++ b/tools/include/xen-sys/Linux/privcmd.h
>>>
>>> @@ -110,6 +110,16 @@ typedef struct privcmd_map_hva_to_gpfns {
>>>
>>>     int add_mapping;
>>>
>>>   } privcmd_map_hva_to_gpfns_t;
>>>
>>>
>>>
>>> +typedef struct privcmd_p2pdma_distance {
>>>
>>> +  __u32 provider_bus;
>>>
>>> +  __u32 provider_slot;
>>>
>>> +  __u32 provider_func;
>>>
>>> +  __u32 client_bus;
>>>
>>> +  __u32 client_slot;
>>>
>>> +  __u32 client_func;
>>>
>>> +  __u32 distance;
>>>
>>> +} privcmd_p2pdma_distance_t;
>>>
>>>
>>>
>>> "Distance" typically is a symmetric thing. Why the asymmetry here? And
>>>
>>> why __u32 when __u8 will be fine for most fields? And where's the segment
>>>
>>> part of the device coordinates? Finally, with it being merely stub
>>>
>>> implementations that you add here, all details on where the needed info
>>>
>>> is to come from are missing.
>>>
>>> "Distance" is p2pdma-distance between two PCI devices, it's calculated in 
>>> kernel driver.I don't get why it's symmetric?
>>
>> Distance from A to B is usually the same as that from B to A. But yes,
>> not necessarily always (thinking of e.g. rings). Yet still I'm unclear
>> about the distinction between provide and client.
> 
> Provider - A driver which provides or publishes P2P resources.
> Client - A driver which makes use of a resource.
> 
> In our case, we want to use passthrough dGPU render data, and virtio 
> iGPU display data. So dGPU need to import display buffer of iGPU. iGPU 
> is provider and dGPU is client.

Right, but: Is this arrangement relevant for the new ioctl? Aren't
you simply after the distance between two devices, of which your
provider/client model is merely a special case?

Jan