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

[Jason Gunthorpe <jgg@ziepe.ca>]

On Wed, May 13, 2026 at 09:18:28PM +0200, Jiri Pirko wrote:
> Tue, May 12, 2026 at 08:12:36PM +0200, jgg@ziepe.ca wrote:
> >On Wed, May 06, 2026 at 04:14:34PM +0200, Jiri Pirko wrote:
> >> Wed, May 06, 2026 at 03:37:57PM +0200, jgg@ziepe.ca wrote:
> >> >On Mon, May 04, 2026 at 03:57:17PM +0200, Jiri Pirko wrote:
> >> >
> >> >> +/**
> >> >> + * ib_umem_get - Canonical on-demand umem getter.
> >> >> + * @device:        IB device.
> >> >> + * @udata:         uverbs udata bundle (may be NULL).
> >> >> + * @attr_id:       per-command UMEM attribute id; consulted if @udata is set.
> >> >> + * @legacy_filler: optional command-specific legacy attr filler.
> >> >> + *                 invoked if @udata is set.
> >> >> + * @va_fallback:   if true, build a VA-typed desc with @addr.
> >> >> + * @addr:          user VA, used if @va_fallback is true.
> >> >> + * @size:          driver-required minimum length.
> >> >> + * @access:        IB access flags forwarded to ib_umem_get_desc().
> >> >> + *
> >> >> + * Return: valid umem on success, ERR_PTR(...) on error, NULL
> >> >> + * if no source produced a buffer (only possible when @va_fallback is false).
> >> >> + */
> >> >> +struct ib_umem *ib_umem_get(struct ib_device *device, struct ib_udata *udata,
> >> >> +			    u16 attr_id,
> >> >> +			    ib_umem_buf_desc_filler_t legacy_filler,
> >> >> +			    bool va_fallback, u64 addr, size_t size, int access)
> >> >
> >> >I didn't try to look at what the drivers actually do, but I'm slightly
> >> >surprised not to see an addr_size here? Is it the case the drivers
> >> >don't have have a uhw->size to go along with their uhw->va?
> >> 
> >> "size_t size". What am I missing?
> >
> >size is the minimum length, not the actual length passed into the
> >system call
> 
> Right, you're correct. Size is the post-pin minimum, and on
> the VA fallback path it serves also as the pin length (we synthesize
> desc.length = size when no attr/legacy filler matched).
> 
> 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?

Jason