Re: [PATCH v6 1/4] RDMA/umem: Support importing dma-buf as user memory region

[Daniel Vetter <daniel@ffwll.ch>]

On Tue, Nov 3, 2020 at 6:36 PM Xiong, Jianxin <jianxin.xiong@intel.com> wrote:
>
>
> > -----Original Message-----
> > From: Daniel Vetter <daniel@ffwll.ch>
> > Sent: Friday, October 23, 2020 6:45 PM
> > To: Jason Gunthorpe <jgg@ziepe.ca>
> > Cc: Xiong, Jianxin <jianxin.xiong@intel.com>; linux-rdma <linux-rdma@vger.kernel.org>; dri-devel <dri-devel@lists.freedesktop.org>; Leon
> > Romanovsky <leon@kernel.org>; Doug Ledford <dledford@redhat.com>; Vetter, Daniel <daniel.vetter@intel.com>; Christian Koenig
> > <christian.koenig@amd.com>
> > Subject: Re: [PATCH v6 1/4] RDMA/umem: Support importing dma-buf as user memory region
> >
> > > > > +
> > > > > +#ifdef CONFIG_DMA_VIRT_OPS
> > > > > +   if (device->dma_device->dma_ops == &dma_virt_ops)
> > > > > +           return ERR_PTR(-EINVAL); #endif
> > > >
> > > > Maybe I'm confused, but should we have this check in dma_buf_attach,
> > > > or at least in dma_buf_dynamic_attach? The p2pdma functions use that
> > > > too, and I can't imagine how zerocopy should work (which is like the
> > > > entire point of
> > > > dma-buf) when we have dma_virt_ops.
> > >
> > > The problem is we have RDMA drivers that assume SGL's have a valid
> > > struct page, and these hacky/wrong P2P sgls that DMABUF creates cannot
> > > be passed into those drivers.
> > >
> > > But maybe this is just a 'drivers are using it wrong' if they call
> > > this function and expect struct pages..
> > >
> > > The check in the p2p stuff was done to avoid this too, but it was on a
> > > different flow.
> >
> > Yeah definitely don't call dma_buf_map_attachment and expect a page back. In practice you'll get a page back fairly often, but I don't think
> > we want to bake that in, maybe we eventually get to non-hacky dma_addr_t only sgl.
> >
> > What I'm wondering is whether dma_buf_attach shouldn't reject such devices directly, instead of each importer having to do that.
>
> Come back here to see if consensus can be reached on who should do the check. My
> thinking is that it could be over restrictive for dma_buf_attach to always reject
> dma_virt_ops. According to dma-buf documentation the back storage would be
> moved to system area upon mapping unless p2p is requested and can be supported
> by the exporter. The sg_list for system memory would have struct page present.

So I'm not clear on what this dma_virt_ops stuff is for, but if it's
an entirely virtual device with cpu access, then you shouldn't do
dma_buf_map_attachment, and then peek at the struct page in the sgl.
Instead you need to use dma_buf_vmap/vunmap and
dma_buf_begin/end_cpu_access. Otherwise the coherency managed is all
potentially busted. Also note that cpu access from the kernel to
dma-buf is a rather niche feature (only some usb device drivers use
it), so expect warts.

If this is the case, then I think dma_buf_attach should check for this
and reject such imports, since that's an importer bug.

If it's otoh something rdma specific, then I guess rdma checking for this is ok.

As a third option, if it's something about the connectivity between
the importing and exporting device, then this should be checked in the
->attach callback the exporter can provide, like the p2p check. The
idea here is that for device specific remapping units (mostly found
ond SoC, and not something like a standard iommu managed by the
dma-api), some of which can even do funny stuff like rotation of 2d
images, can be access by some, but not other. And only the exporter is
aware of these restrictions.

Now I dunno which case this one here is.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch