bus_dmamap_load_uio() and user data

[Mark Tinguely]

>  You should use the pmap from the thread in the uio structure.  Similar to
>  this from the x86 bus_dma code:
>
>          if (uio->uio_segflg == UIO_USERSPACE) {
>                  KASSERT(uio->uio_td != NULL,
>                          ("bus_dmamap_load_uio: USERSPACE but no proc"));
>                  pmap = vmspace_pmap(uio->uio_td->td_proc->p_vmspace);
>          } else
>                  pmap = NULL;
>
>  Later when doing VA -> PA conversions the code does this:
>
>                          if (pmap)
>                                  paddr = pmap_extract(pmap, vaddr);
>                          else
>                                  paddr = pmap_kextract(vaddr);
>

We do that, but I notice that all the architecture that implement
bounce buffers assume the VA is in the current map. Most of the
addresses are KVA, but bus_dmamap_load_uio() can be in the user space.

I was wondering about the sequence:

 bus_dmamap_load_uio() user space
   dma_load_buffer()
     add bounce page save UVA (in caller user map)

later:

 bus_dma_sync
   copies bounce buffer from saved UVA. <- here is my concern. The user pmap
	is not remembered use current pmap.

Since the bounce buffer copy routines have been running in other architectures
for years without corruption, I was wondering we can safely assume that the
dma sync is running in the same thread/address space as the bus_dmamap_load_uio
call. I was hoping you would say, don't worry the scheduler would always
reload the same thread to execute the dma sync code ...

Thank-you,

--Mark Tinguely