Re: [RFC 4/4] rpmsg: DMA map sgs passed to virtio

["Michael S. Tsirkin" <mst@redhat.com>]

On Sat, May 16, 2015 at 12:32:10PM +0300, Ohad Ben-Cohen wrote:
> On Thu, May 7, 2015 at 3:28 AM, Edgar E. Iglesias
> <edgar.iglesias@gmail.com> wrote:
> >
> > On Wed, May 06, 2015 at 03:51:48PM +0930, Rusty Russell wrote:
> > > "Edgar E. Iglesias" <edgar.iglesias@gmail.com> writes:
> > > > From: "Edgar E. Iglesias" <edgar.iglesias@xilinx.com>
> > > >
> > > > Signed-off-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
> > >
> > > First off, I have handed maintainership off to Michael S. Tsirkin, so
> > > his word is now law.
> > >
> > > That said... there's nothing fundamentally *wrong* with this, but it's
> > > not how standard virtio works.  We decided some time ago that as we're
> > > paravirtualized, we would not be doing address mapping.
> > >
> > > rpmsg uses virtio, but it's with a twist: they're not talking to a
> > > host.  Thus my preference, in order, would be:
> > >
> > > 1) Don't use non-kmalloc addresses.
> > > 2) If that's not possible, call these _dma interfaces _rpmsg instead,
> > >    so normal virtio users don't get confused and try to use them.
> >
> > Thanks Rusty,
> >
> > That was helpful, I'll see if I can do something in line with nr 2.
> >
> > AFAICT, #1 will be hard. The remote-processor would have to be
> > cache-coherent and share memory address-space view with the master
> > CPU. This is not the common case for remoteproc (unlike when virtio
> > communication flows between host and guest on the same CPU or SMP system).
> > Ohad, do you have any thoughts on this?
> 
> rpmsg is allocating a large chunk (256KB) of physically-contiguous CMA
> memory today, which is exposed via the dma_alloc_coherent API (and set
> up in advance by platform-specific code), so if #2 above is
> acceptable, it would be easier, yeah.
> 
> Thanks,
> Ohad.

I'm thinking same as Rusty: I'd prefer 1 but if not possible, I can live
with 2 above.

-- 
MST