Re: [PATCH v1 1/5] IB/uverbs: ex_query_device: answer must not depend on request's comp_mask

[Haggai Eran <haggaie-VPRAkNaXOzVWk0Htik3J/w@public.gmane.org>]

On 30/01/2015 19:26, Yann Droneaud wrote:
> Hi,
> 
> Le jeudi 29 janvier 2015 à 15:17 -0800, Roland Dreier a écrit :
>> On Thu, Jan 29, 2015 at 1:59 PM, Yann Droneaud <ydroneaud-RlY5vtjFyJ3QT0dZR+AlfA@public.gmane.org> wrote:
>>>> Roland: I agree with Yann, these patches need to go in, or the ODP
>>>> patches reverted.
>>
>>> Reverting all On Demand Paging patches seems overkill:
>>> if something as to be reverted it should be commit 5a77abf9a97a
>>> ("IB/core: Add support for extended query device caps") and the part of
>>> commit 860f10a799c8 ("IB/core: Add flags for on demand paging support")
>>> which modify ib_uverbs_ex_query_device().
>>
>> Thank you and Jason for taking on this interface.
>>
>> At this point I feel like I do about the IPoIB changes -- we should
>> revert the broken stuff and get it right for 3.20.
>>
>> If we revert the two things you describe above, is everything else OK
>> to leave in 3.19 with respect to ABI?
>>
> 
> I've tried to review every changes since v3.18 on drivers/infiniband
> include/rdma and include/uapi/rdma with respect to ABI issues.
> 
> I've noticed no other issue, but I have to admit I've not well reviewed
> the drivers (hw/) internal changes.
> 
> If the IB_USER_VERBS_EX_CMD_QUERY_DEVICE and ib_uverbs_ex_query_device
> changes are going to be reverted for v3.19, the on-demand-paging
> feature will be available (IB_DEVICE_ON_DEMAND_PAGING will be set 
> device_cap_flags in response to non extended QUERY_DEVICE for mlx5 HCA
> and IB_ACCESS_ON_DEMAND access flag will be effective for REG_MR 
> uverbs), but its parameters won't be. 

For user-space to make use of on demand paging, it should verify the
specific transport and operation is supported. If they don't, they will
encounter errors when a page fault occurs.

> I don't know if it's a no-go for 
> the usage of on-demand paging by userspace: I have not the chance
> of owning HCA with the support for this feature, nor the patches
> libibverbs / libmlx5 ... (anyway I would not have the time to test).
> I've hoped people from Mellanox would have commented on the revert 
> option too.

I would prefer it if Yann's patches are accepted. I understand it is
very late, but they are quite short, and I think they provide the right
semantic for this new verb.

As a second option, in case you prefer to revert the extended query
device patch, I will send a patch shortly to do that.

Regards,
Haggai