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

[Christoph Hellwig <hch@lst.de>]

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.