linux-kernel - Re: [RFC PATCH 20/28] IB/core: Introduce API for initializing a RW ctx from a DMA address

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Date:   Thu, 20 Jun 2019 14:11:05 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Logan Gunthorpe <logang@...tatee.com>
Cc:     linux-kernel@...r.kernel.org, linux-block@...r.kernel.org,
        linux-nvme@...ts.infradead.org, linux-pci@...r.kernel.org,
        linux-rdma@...r.kernel.org, Jens Axboe <axboe@...nel.dk>,
        Christoph Hellwig <hch@....de>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Sagi Grimberg <sagi@...mberg.me>,
        Keith Busch <kbusch@...nel.org>,
        Stephen Bates <sbates@...thlin.com>
Subject: Re: [RFC PATCH 20/28] IB/core: Introduce API for initializing a RW
 ctx from a DMA address

On Thu, Jun 20, 2019 at 10:59:44AM -0600, Logan Gunthorpe wrote:
> 
> 
> On 2019-06-20 10:49 a.m., Jason Gunthorpe wrote:
> > On Thu, Jun 20, 2019 at 10:12:32AM -0600, Logan Gunthorpe wrote:
> >> Introduce rdma_rw_ctx_dma_init() and rdma_rw_ctx_dma_destroy() which
> >> peform the same operation as rdma_rw_ctx_init() and
> >> rdma_rw_ctx_destroy() respectively except they operate on a DMA
> >> address and length instead of an SGL.
> >>
> >> This will be used for struct page-less P2PDMA, but there's also
> >> been opinions expressed to migrate away from SGLs and struct
> >> pages in the RDMA APIs and this will likely fit with that
> >> effort.
> >>
> >> Signed-off-by: Logan Gunthorpe <logang@...tatee.com>
> >>  drivers/infiniband/core/rw.c | 74 ++++++++++++++++++++++++++++++------
> >>  include/rdma/rw.h            |  6 +++
> >>  2 files changed, 69 insertions(+), 11 deletions(-)
> >>
> >> diff --git a/drivers/infiniband/core/rw.c b/drivers/infiniband/core/rw.c
> >> index 32ca8429eaae..cefa6b930bc8 100644
> >> +++ b/drivers/infiniband/core/rw.c
> >> @@ -319,6 +319,39 @@ int rdma_rw_ctx_init(struct rdma_rw_ctx *ctx, struct ib_qp *qp, u8 port_num,
> >>  }
> >>  EXPORT_SYMBOL(rdma_rw_ctx_init);
> >>  
> >> +/**
> >> + * rdma_rw_ctx_dma_init - initialize a RDMA READ/WRITE context from a
> >> + *	DMA address instead of SGL
> >> + * @ctx:	context to initialize
> >> + * @qp:		queue pair to operate on
> >> + * @port_num:	port num to which the connection is bound
> >> + * @addr:	DMA address to READ/WRITE from/to
> >> + * @len:	length of memory to operate on
> >> + * @remote_addr:remote address to read/write (relative to @rkey)
> >> + * @rkey:	remote key to operate on
> >> + * @dir:	%DMA_TO_DEVICE for RDMA WRITE, %DMA_FROM_DEVICE for RDMA READ
> >> + *
> >> + * Returns the number of WQEs that will be needed on the workqueue if
> >> + * successful, or a negative error code.
> >> + */
> >> +int rdma_rw_ctx_dma_init(struct rdma_rw_ctx *ctx, struct ib_qp *qp,
> >> +		u8 port_num, dma_addr_t addr, u32 len, u64 remote_addr,
> >> +		u32 rkey, enum dma_data_direction dir)
> > 
> > Why not keep the same basic signature here but replace the scatterlist
> > with the dma vec ?
> 
> Could do. At the moment, I had no need for dma_vec in this interface.

I think that is because you only did nvme not srp/iser :)

> >> +{
> >> +	struct scatterlist sg;
> >> +
> >> +	sg_dma_address(&sg) = addr;
> >> +	sg_dma_len(&sg) = len;
> > 
> > This needs to fail if the driver is one of the few that require
> > struct page to work..
> 
> Yes, right. Currently P2PDMA checks for the use of dma_virt_ops. And
> that probably should also be done here. But is that sufficient? You're
> probably right that it'll take an audit of the RDMA tree to sort that out.

For this purpose I'd be fine if you added a flag to the struct
ib_device_ops that is set on drivers that we know are OK.. We can make
that list bigger over time.

> > This is not so hard to do, as most drivers are already struct page
> > free, but is pretty much blocked on needing some way to go from the
> > block layer SGL world to the dma vec world that does not hurt storage
> > performance.
> 
> Maybe I can end up helping with that if it helps push the ideas here
> through. (And assuming people think it's an acceptable approach for the
> block-layer side of things).

Let us hope for a clear decision then

Jason