Re: [PATCH rdma-next v3 03/17] RDMA/core: Introduce generic buffer descriptor infrastructure for umem

[Jiri Pirko <jiri@resnulli.us>]

Thu, May 14, 2026 at 02:14:10PM +0200, jgg@ziepe.ca wrote:
>On Thu, May 14, 2026 at 11:02:45AM +0200, Jiri Pirko wrote:
>> >> There's no addr_size because no caller has a user-passed
>> >> length distinct from the driver-required minimum.
>> >> Drivers either have no length in their legacy ucmd (mlx4/mlx5 CQ, mlx5 QP,
>> >> the size is driver-computed from entries*cqe_size etc.) or they
>> >> pass ucmd.buf_size which serves as both (vmw_pvrdma, qedr, mlx5
>> >> SRQ, ...).
>> >
>> >This is what I was wondering about, so how does it work when there is
>> >a ucmd.buf_size and also a minimum size computed from
>> >entries*cqe_size? What does the driver write?
>> >
>> >I think the driver has to pass in both the minimum size computed from
>> >entries*cqe_size and also the uhw exact size? The minimum size is used
>> >if the uhw path isn't used to check the other attribute while the uhw
>> >exact size is used to pin the memory if the uhw path is used - and it
>> >should also be checked against the minimum?
>> 
>> I asked Claude to check every caller: At the moment of conversion
>> none of them would have addr_size != min_size. They split into
>> three groups:
>> 
>>   1) Driver-computed total only (no user-passed length distinct from
>>      min). addr_size == min_size by construction:
>> 
>>        mlx4 CQ/QP/SRQ           entries * cqe_size, qp->buf_size, ...
>>        mlx5 CQ/QP/SRQ           entries * cqe_size, rwq->buf_size, ...
>>        bnxt_re QP/SRQ/CQ-resize max_wqe * wqe_size, entries * sizeof
>>        mana CQ                  cq->cqe * COMP_ENTRY_SIZE
>>        hns_roce MTR             mtr_bufs_size(buf_attr)
>>        qedr SRQ producer pair   sizeof(struct rdma_srq_producers)
>>        all DBR helpers          PAGE_SIZE
>
>OK these make sense
>
>>   2) MR registration / opaque user umem. The user-passed length *is*
>>      the request; there's no separate driver minimum. addr_size ==
>>      min_size by definition:
>> 
>>        reg_user_mr in every driver
>>        mlx5 devx user umem
>
>MR has to use the exact size passed in the top level system call, so
>it probably needs some special helper that does that instead of minimum
> 
>>   3) User-passed length without a driver minimum cross-check today:
>> 
>>        vmw_pvrdma CQ/QP/SRQ     derivable min (entries*sizeof, wqe_*),
>>                                 not computed in the user path
>>        qedr CQ/QP/SRQ           ureq.size, no min computed in wrapper
>>        mana WQ, QP raw_sq,
>>             QP RC queues        ucmd.{wq,sq}_buf_size, queue_size[],
>>                                 no derivable min
>>        ionic CQ, QP sq/rq       req_cq->size, sq->size, rq->size,
>>                                 no derivable min
>
>Yeah, these are exactly the ones I'd expect to have a second
>parameter. Something like ucmd.wq_buf_size should be entirely ignored
>if the user passes a new attribute, not silently used as a minimum
>check. That logic has to be done in the helper
>
>So you imagine another helper for these four drivers with an
>additional parameter?
>
>> Given that, I'd prefer to keep the single size argument for now and
>> spell the contract out in the kdoc:
>> 
>>   @size: minimum required umem length, validated post-pin against any
>>          descriptor produced via @attr_id / @legacy_filler; also used
>>          as the pin length on the VA fallback path. Callers that have
>>          a distinct user-passed length must validate it against their
>>          driver minimum before calling.
>
>> If/when a driver actually needs distinct values, splitting into
>> addr_size + min_size is mechanical.
>
>Ok, maybe mention in the commit message this has trouble for the four
>drivers in group 3

I split the "size" arg into 2 for ib_umem_get(), documented and noted
that in future the new helper working with both arg is going to be
needed for group 3 drivers.