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

[Christian König]

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 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.

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.

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