Re: Re: [PATCH 2/5] RDMA/core: remove use of dma_virt_ops

["Bernard Metzler" <BMT@zurich.ibm.com>]

-----"Christoph Hellwig" <hch@lst.de> wrote: -----

>To: "Jason Gunthorpe" <jgg@ziepe.ca>
>From: "Christoph Hellwig" <hch@lst.de>
>Date: 11/04/2020 03:02PM
>Cc: "Christoph Hellwig" <hch@lst.de>, "Bjorn Helgaas"
><bhelgaas@google.com>, "Logan Gunthorpe" <logang@deltatee.com>,
>linux-rdma@vger.kernel.org, linux-pci@vger.kernel.org,
>iommu@lists.linux-foundation.org
>Subject: [EXTERNAL] Re: [PATCH 2/5] RDMA/core: remove use of
>dma_virt_ops
>
>On Wed, Nov 04, 2020 at 09:42:41AM -0400, Jason Gunthorpe wrote:
>> On Wed, Nov 04, 2020 at 10:50:49AM +0100, Christoph Hellwig wrote:
>> 
>> > +int ib_dma_virt_map_sg(struct ib_device *dev, struct scatterlist
>*sg, int nents)
>> > +{
>> > +	struct scatterlist *s;
>> > +	int i;
>> > +
>> > +	for_each_sg(sg, s, nents, i) {
>> > +		sg_dma_address(s) = (uintptr_t)sg_virt(s);
>> > +		sg_dma_len(s) = s->length;
>> 
>> Hmm.. There is nothing ensuring the page is mapped here for this
>> sg_virt(). Before maybe some of the kconfig stuff prevented highmem
>> systems indirectly, I wonder if we should add something more direct
>to
>> exclude highmem for these drivers?
>
>I had actually noticed this earlier as well and then completely
>forgot
>about it..
>
>rdmavt depends on X86_64, so it can't be used with highmem, but for
>rxe and siw there weren't any such dependencies so I think we were
>just
>lucky.  Let me send a fix to add explicit depencies and then respin
>this
>series on top of that..
>
>> Sigh. I think the proper fix is to replace addr/length with a
>> scatterlist pointer in the struct ib_sge, then have SW drivers
>> directly use the page pointer properly.
>
>The proper fix is to move the DMA mapping into the RDMA core, yes.
>And as you said it will be hard.  But I don't think scatterlists
>are the right interface.  IMHO we can keep re-use the existing
>struct ib_sge:
>
>struct ib_ge {
>	u64     addr;
>	u32     length;
>	u32     lkey;
>};
>
>with the difference that if lkey is not a MR, addr is the physical
>address of the memory, not a dma_addr_t or virtual address.
>

lkey of zero to pass a physical buffer, only allowed for
kernel applications? Very nice idea I think.

btw.
It would even get the vain blessing of the old IETF RDMA
verbs draft ;)

https://tools.ietf.org/html/draft-hilland-rddp-verbs-00#page-90


(section '7.2.1 STag of zero' - read lkey for STag)