Re: [PATCH RFC 06/12] remoteproc: virtio: add bounce buffering for data buffers

[Francesco Valla <francesco@valla.it>]

Hi Robin, Mathieu,

On Fri, Sep 25, 2026 at 05:48:25PM +0100, Robin Murphy wrote:
> On 25/09/2026 4:07 pm, Mathieu Poirier wrote:
> > On Wed, Sep 23, 2026 at 06:05:35PM +0200, Francesco Valla wrote:
> > > On Wed, Sep 23, 2026 at 08:44:41AM -0600, Mathieu Poirier wrote:
> > > > On Tue, 22 Sept 2026 at 13:39, Francesco Valla <francesco@valla.it> wrote:
> > > > > 
> > > > > On Tue, Sep 22, 2026 at 09:58:53AM -0600, Mathieu Poirier wrote:
> > > > > > On Wed, Sep 16, 2026 at 11:10:51PM +0200, Francesco Valla wrote:
> > > > > > > Depending on the driver originating them, data buffers used for virtio
> > > > > > > communication can either:
> > > > > > > 
> > > > > > >    - already be allocated from the coherent memory area that is
> > > > > > >      accessible by the remote processor; this is the case of rpmsg
> > > > > > >      and the rproc flavor of virtio-console;
> > > > > > >    - be allocated from generic kmem, and thus not accessible directly by
> > > > > > >      the remote processor.
> > > > > > > 
> > > > > > > Exploiting the map operations, which are used by the virtio framework
> > > > > > > when VIRTIO_F_ACCESS_PLATFORM is part of a vdev's feature flags, add
> > > > > > > bounce buffering for the second case: when the map() callback is called
> > > > > > > for a buffer, one or more pages of coherent memory are allocated and
> > > > > > > data is copied to them, then they are exposed to the remote processor;
> > > > > > > the data is  then bounced back on unmap().
> > > > > > > 
> > > > > > > The first case is not impacted, since buffers already suitable for
> > > > > > > remote transmission are passed through.
> > > > > > > 
> > > > > > > With the bounce buffering in place, any kind of virtio device can be
> > > > > > > supported through the remoteproc-virtio transport, at least from a
> > > > > > > data exchange standpoint.
> > > > > > 
> > > > > > Is this _necessary_ for the imx93 platform you are implementing feature for?
> > > > > > 
> > > > > 
> > > > > If I don't want to fundamentally change how the remoteproc integration
> > > > > works (i.e.: using buffers only from a pre-shared area), yes. While in
> > > > > my test environment the Cortex-M33 serving as remoteproc is able to
> > > > > access the whole RAM space, that is not always the case.
> > > > 
> > > > The first sentence tells me it is mandatory while the second says it
> > > > is not.  I understand the use case but don't want to bloat the
> > > > subsystem with code that is trying to address a problem you currently
> > > > don't have.
> > > > 
> > > 
> > > Let me rephrase: while on i.MX93 the Cortex-M33 can theoretically access
> > > the whole RAM space, that is not a good idea from a security point of
> > > view and can be the source of a number of bugs. The target is to
> > > statically define a static shared memory area (as I am doing on i.MX95)
> > > and only use that.
> > 
> > As Robin pointed out, have you looked at using a restricted-dma-pool for that?
> > Note that I am not familiar with the concept but open to go that way if it can
> > work for us.
> > 
> > Robin, can you point us to a simple example we could look at?
> The only in-tree example is mt8192-asurada.dtsi, but even there the
> fundamental principle seems exactly the same - the system interconnect is
> locked down such that there's only a particular region of "shared" memory
> that PCIe DMA can access, so the restricted pool is placed there, and in
> that case can occupy the entire region since the wifi adapter only really
> does streaming DMA - restricted pools have some limited ability to act as a
> fallback for coherent allocations, which won't work for everything, but does
> happen to be enough for that wifi driver.
> 
> Here it would be a case of reserving some of the shared region for a
> restricted pool alongside the "vdevbuffer" coherent pool, adding it to the
> memory-region list of the relevant device(s), and usually that would then
> just work, since the setup is all done automatically by the core DT code.
> However I know remoteproc does some funky stuff with child devices, so it's
> quite possible there might need to be something more done there. But still
> far, far less than reimplementing a whole other bounce-buffering system.
> 
> Thanks,
> Robin.

I took a look at the restricted-memory-pool - with which I wasn't
familiar - and seems exactly what is needed here.

I am working on a new prototype with it - if everything works as
expected (I still did not have time to try it) it should only need some
very limited glue code, cutting the modifications in the
remoteproc_virtio driver by 95%. I'll let you knwow.

Thank you for the hint!

Regards,
Francesco