On Mon, Jan 14, 2019 at 11:53:08AM -0700, Jason Gunthorpe wrote:
> 'coherent DMA' memory is a property of the cache and when it gets
> remapped to userspace the special cache properties get lost.
That is not the case. There are a few ways to implement coherent
memory, with the most common ones being:
a) do nothing (the x86 way) - this will just work from userspace
b) remap the memory with an uncached bit in the PTE. dma_mmap_coherent
will ensure this bit is also set for the userspace mapping.
Everything will just work
c) allocate from a special pool that is set uncached by the firmware/
hardware at the physical address level. This will just work
again.
> > Also, if the streaming DMA mapping is used, how can the userspace
> > call dma_sync_*? As far as I can tell, the userspace
> > udma_*_device_barrier doesn't seem to replace it.
>
> It can't. platforms that require privileged functionality to do
> dma_sync are not supported by RDMA. Other platforms are required to
> implement their functionality via barriers.
While there might be a few architectures were cache invalidation /
writeback is privileged this is not the common case as far as I can
tell. In general those are available to userspace code as well.
