-----"Christoph Hellwig" <hch@xxxxxx> wrote: -----
>To: "Jason Gunthorpe" <jgg@xxxxxxxx>
>From: "Christoph Hellwig" <hch@xxxxxx>
>Date: 11/04/2020 03:02PM
>Cc: "Christoph Hellwig" <hch@xxxxxx>, "Bjorn Helgaas"
><bhelgaas@xxxxxxxxxx>, "Logan Gunthorpe" <logang@xxxxxxxxxxxx>,
>linux-rdma@xxxxxxxxxxxxxxx, linux-pci@xxxxxxxxxxxxxxx,
>iommu@xxxxxxxxxxxxxxxxxxxxxxxxxx
>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)
