[Intel-gfx] [PATCH 1/2] drm/vgem: Do not allocate backing shmemfs file for an import dmabuf object

[Daniel Vetter]

On Wed, Jul 08, 2020 at 11:22:00AM +0200, Christian König wrote:
> Am 07.07.20 um 20:35 schrieb Chris Wilson:
> > Quoting lepton (2020-07-07 19:17:51)
> > > On Tue, Jul 7, 2020 at 10:20 AM Chris Wilson <chris at chris-wilson.co.uk> wrote:
> > > > Quoting lepton (2020-07-07 18:05:21)
> > > > > On Tue, Jul 7, 2020 at 9:00 AM Chris Wilson <chris at chris-wilson.co.uk> wrote:
> > > > > > If we assign obj->filp, we believe that the create vgem bo is native and
> > > > > > allow direct operations like mmap() assuming it behaves as backed by a
> > > > > > shmemfs inode. When imported from a dmabuf, the obj->pages are
> > > > > > not always meaningful and the shmemfs backing store misleading.
> > > > > > 
> > > > > > Note, that regular mmap access to a vgem bo is via the dumb buffer API,
> > > > > > and that rejects attempts to mmap an imported dmabuf,
> > > > > What do you mean by "regular mmap access" here?  It looks like vgem is
> > > > > using vgem_gem_dumb_map as .dumb_map_offset callback then it doesn't call
> > > > > drm_gem_dumb_map_offset
> > > > As I too found out, and so had to correct my story telling.
> > > > 
> > > > By regular mmap() access I mean mmap on the vgem bo [via the dumb buffer
> > > > API] as opposed to mmap() via an exported dma-buf fd. I had to look at
> > > > igt to see how it was being used.
> > > Now it seems your fix is to disable "regular mmap" on imported dma buf
> > > for vgem. I am not really a graphic guy, but then the api looks like:
> > > for a gem handle, user space has to guess to find out the way to mmap
> > > it. If user space guess wrong, then it will fail to mmap. Is this the
> > > expected way
> > > for people to handle gpu buffer?
> > You either have a dumb buffer handle, or a dma-buf fd. If you have the
> > handle, you have to use the dumb buffer API, there's no other way to
> > mmap it. If you have the dma-buf fd, you should mmap it directly. Those
> > two are clear.
> > 
> > It's when you import the dma-buf into vgem and create a handle out of
> > it, that's when the handle is no longer first class and certain uAPI
> > [the dumb buffer API in particular] fail.
> > 
> > It's not brilliant, as you say, it requires the user to remember the
> > difference between the handles, but at the same time it does prevent
> > them falling into coherency traps by forcing them to use the right
> > driver to handle the object, and have to consider the additional ioctls
> > that go along with that access.
> 
> Yes, Chris is right. Mapping DMA-buf through the mmap() APIs of an importer
> is illegal.
> 
> What we could maybe try to do is to redirect this mmap() API call on the
> importer to the exporter, but I'm pretty sure that the fs layer wouldn't
> like that without changes.

We already do that, there's a full helper-ified path from I think shmem
helpers through prime helpers to forward this all. Including handling
buffer offsets and all the other lolz back&forth.

Of course there's still the problem that many drivers don't forward the
cache coherency calls for begin/end cpu access, so in a bunch of cases
you'll cache cacheline dirt soup. But that's kinda standard procedure for
dma-buf :-P

But yeah trying to handle the mmap as an importer, bypassing the export:
nope. The one exception is if you have some kind of fancy gart with
cpu-visible pci bar (like at least integrated intel gpus have). But in
that case the mmap very much looks&acts like device access in every way.

Cheers, Daniel

> Regards,
> Christian.
> 
> 
> > -Chris
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch