Re: [PATCH] lib/scatterlist: Provide a DMA page iterator

Thomas Hellstrom
2019-01-17T10:47:38Z

On Thu, 2019-01-17 at 10:30 +0100, hch@lst.de wrote:
> On Wed, Jan 16, 2019 at 10:24:36AM -0700, Jason Gunthorpe wrote:
> > The fact is there is 0 industry interest in using RDMA on platforms
> > that can't do HW DMA cache coherency - the kernel syscalls required
> > to
> > do the cache flushing on the IO path would just destroy performance
> > to
> > the point of making RDMA pointless. Better to use netdev on those
> > platforms.
>
> In general there is no syscall required for doing cache flushing, you
> just issue the proper instructions directly from userspace.

But what if there are other coherence issues? Like bounce-buffers?
I'd like to +1 on what Jason says about industry interest: FWIW, vmwgfx
is probably one of the graphics drivers that would lend itself best to
do a fully-dma-interface compliant graphics stack experiment. But being
a paravirtual driver, all platforms we can ever run on are fully
coherent unless someone introduces a fake incoherency by forcing
swiotlb. Putting many man-months of effort into supporting systems on
which we would never run on and can never test on can never make more
than academic sense.

>
>
> > The reality is that *all* the subsytems doing DMA kernel bypass are
> > ignoring the DMA mapping rules, I think we should support this
> > better,
> > and just accept that user space DMA will not be using syncing.
> > Block
> > access in cases when this is required, otherwise let it work as is
> > today.
>
> In that case we just need to block userspace DMA access entirely.
> Which given the amount of problems it creates sounds like a pretty
> good idea anyway.

I'm not sure I'm following you here. Are you suggesting scratching
support for all (GP)GPU- and RDMA drivers?

Thanks,
Thomas