Re: [EXTERNAL] Re: [PATCH rdma-next 1/1] RDMA/mana_ib: indicate that inline data is not supported

[Jason Gunthorpe <jgg@nvidia.com>]

On Thu, Jul 18, 2024 at 03:05:59PM +0000, Konstantin Taranov wrote:
> > > > > Yes, you are. If user asked for specific functionality
> > > > > (max_inline_data != 0) and your device doesn't support it, you should
> > return an error.
> > > > >
> > > > > pvrdma, mlx4 and rvt are not good examples, they should return an
> > > > > error as well, but because of being legacy code, we won't change them.
> > > > >
> > > > > Thanks
> > > > >
> > > >
> > > > I see. So I guess we can return a larger value, but not smaller. Right?
> > > > I will send v2 that fails QP creation then.
> > > >
> > > > In this case, may I submit a patch to rdma-core that queries device
> > > > caps before trying to create a qp in rdma_client.c and
> > > > rdma_server.c? As that code violates what you described.
> > >
> > > Let's ask Jason, why is that? Do we allow to ignore max_inline_data?
> > >
> > > librdmacm/examples/rdma_client.c
> > >   63         memset(&attr, 0, sizeof attr);
> > >   64         attr.cap.max_send_wr = attr.cap.max_recv_wr = 1;
> > >   65         attr.cap.max_send_sge = attr.cap.max_recv_sge = 1;
> > >   66         attr.cap.max_inline_data = 16;
> > >   67         attr.qp_context = id;
> > >   68         attr.sq_sig_all = 1;
> > >   69         ret = rdma_create_ep(&id, res, NULL, &attr);
> > >   70         // Check to see if we got inline data allowed or not
> > >   71         if (attr.cap.max_inline_data >= 16)
> > >   72                 send_flags = IBV_SEND_INLINE;
> > >   73         else
> > >   74                 printf("rdma_client: device doesn't support
> > IBV_SEND_INLINE, "
> > >   75                        "using sge sends\n");
> > 
> > I think the idea expressed in this code is that if max_inline_data requested
> > too much it would be limited to the device capability.
> > 
> > ie qp creation should limit the requests values to what the HW can do, similar
> > to how entries and other work.
> > 
> > If the HW has no support it should return - for max_inline_data not an error,
> > I guess?
> 
> Yes, this code implies that max_inline_data can be ignored at creation, while the manual of ibv_create_qp says:
> "The function ibv_create_qp() will update the qp_init_attr->cap struct with the actual QP values of the QP that was created;
> the values will be **greater than or equal to** the values
> requested."

Ah, well that seems to be some misunderstandings then, yes.

> I see two options:
> 1) Remove code from rdma examples that rely on ignoring max_inline; add a warning to libibverbs when drivers ignore that value.
> 2) Add to manual that max_inline_data might be ignored by drivers; and allow my current patch that ignores max_inline_data in mana_ib.

I don't know, what do the majority of drivers do? If enough are
already doing 1 then lets force everyone into 1, otherwise we have to
document 2.

And a pyverbs test should be added to cover this weirdness

Jason