[RFC 2/6] udmabuf: add ability to set access flags on udmabuf

[Gerd Hoffmann]

On Fri, Aug 02, 2019 at 09:45:15AM -0700, Gurchetan Singh wrote:
> On Wed, Jul 31, 2019 at 11:40 PM Gerd Hoffmann <kraxel at redhat.com> wrote:
> >
> > On Wed, Jul 31, 2019 at 07:25:13PM -0700, Gurchetan Singh wrote:
> > > The main use for udmabuf is sending guest memory pages
> > > to the host.
> > >
> > > It's generally a bad idea to have to separate mappings with
> > > different attributes. For example, a WC mapping the guest
> > > kernel and cached mapping on the host is problematic.
> > >
> > > Add creation time flags so the user has responsibility for
> > > the specific use case.
> >
> > > -#define UDMABUF_FLAGS_CLOEXEC        0x01
> > > +#define UDMABUF_FLAGS_CLOEXEC    0x01
> > > +#define UDMABUF_FLAGS_PROT_NONE  0x02
> > > +#define UDMABUF_FLAGS_PROT_READ  0x04
> > > +#define UDMABUF_FLAGS_PROT_WRITE 0x08
> >
> > [ didn't look at followup patches yet ]
> >
> > You can't have readonly/writeonly dmabufs.
> > So that isn't going to fly.
> >
> > The commit message suggests this is for cache attributes not protection,
> > so having the flags might make sense, but please don't name the flags
> > PROT_* then.
> 
> Okay, I'll change the flags to CACHED / UNCACHED / WRITE_COMBINE (like
> msm_drm.h).  And since the dma api doesn't work on x86 [1], we'll have
> to call drm_cflush_pages in the guest.  Since caching is privileged on
> ARM and not on x86, that *should* get us write-combine guest buffers.
> 
> [1] https://lists.freedesktop.org/archives/dri-devel/2019-August/229161.html

Ah, so you are aware of the vgem cache synchronization patches.

It's probably a good idea to wait until that is finally settled before
following with udmabuf.

cheers,
  Gerd