On Wed, Jul 8, 2020 at 4:37 PM Christian König <christian.koenig@xxxxxxx> wrote: > > Am 08.07.20 um 11:54 schrieb 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@xxxxxxxxxxxxxxxxxx> wrote: > >>>>> Quoting lepton (2020-07-07 18:05:21) > >>>>>> On Tue, Jul 7, 2020 at 9:00 AM Chris Wilson <chris@xxxxxxxxxxxxxxxxxx> 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. > > Oh, that most likely won't work correctly with unpinned DMA-bufs and > needs to be avoided. > > Each file descriptor is associated with an struct address_space. And > when you mmap() through the importer by redirecting the system call to > the exporter you end up with the wrong struct address_space in your VMA. > > That in turn can go up easily in flames when the exporter tries to > invalidate the CPU mappings for its DMA-buf while moving it. > > Where are we doing this? My last status was that this is forbidden. Hm I thought we're doing all that already, but looking at the code again we're only doing this when opening a new drm fd or dma-buf fd. So the right file->f_mapping is always set at file creation time. And we indeed don't frob this more when going another indirection ... Maybe we screwed up something somewhere :-/ Also I thought the mapping is only taken after the vma is instatiated, otherwise the tricks we're playing with dma-buf already wouldn't work: dma-buf has the buffer always at offset 0, whereas gem drm_fd mmap has it somewhere else. We already adjust vma->vm_pgoff, so I'm wondering whether we could adjust vm_file too. Or is that the thing that's forbidden? -Daniel > Christian. > > > 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
