[Intel-gfx] [PATCH] drm/i915: Introduce mapping of user pages into video memory (userptr) ioctl

[Chris Wilson]

On Mon, Apr 08, 2013 at 09:24:58PM +0200, Daniel Vetter wrote:
> On Mon, Apr 8, 2013 at 7:40 PM, Chris Wilson <chris at chris-wilson.co.uk> wrote:
> > On Mon, Apr 08, 2013 at 07:18:11PM +0200, Daniel Vetter wrote:
> >> On Tue, Feb 12, 2013 at 02:17:22PM +0000, Chris Wilson wrote:
> >> > By exporting the ability to map user address and inserting PTEs
> >> > representing their backing pages into the GTT, we can exploit UMA in order
> >> > to utilize normal application data as a texture source or even as a
> >> > render target (depending upon the capabilities of the chipset). This has
> >> > a number of uses, with zero-copy downloads to the GPU and efficient
> >> > readback making the intermixed streaming of CPU and GPU operations
> >> > fairly efficient. This ability has many widespread implications from
> >> > faster rendering of client-side software rasterisers (chromium),
> >> > mitigation of stalls due to read back (firefox) and to faster pipelining
> >> > of texture data (such as pixel buffer objects in GL or data blobs in CL).
> >> >
> >> > v2: Compile with CONFIG_MMU_NOTIFIER
> >> > v3: We can sleep while performing invalidate-range, which we can utilise
> >> > to drop our page references prior to the kernel manipulating the vma
> >> > (for either discard or cloning) and so protect normal users.
> >> > v4: Only run the invalidate notifier if the range intercepts the bo.
> >> > v5: Prevent userspace from attempting to GTT mmap non-page aligned buffers
> >> >
> >> > Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> >>
> >> Quick bikeshed:
> >> - Still not really in favour of the in-page gtt offset handling ... I
> >>   still think that this is just a fancy mmap api, and it better reject
> >>   attempts to not map anything aligned to a full page outright.
> >
> > Strongly disagree.
> 
> Ok, let's dig out the beaten arguments here ;-)
> - Imo the gtt_offset frobbery is a bit fragile, and getting this right
> in the face of ppgtt won't make it better. And yes the only reason we
> still have that field is that you've shot down any patch to remove it
> citing userptr here. So "it's here already" doesn't count ;-)
> - Userptr for i915 is an mmap interface, and that works on pages,
> lying to userspace isn't great.

No. Due to the nature of existing decades old userspace, I need to map
byte ranges, so I do not view this as a simple equivalence to mmapping
a bo.

> - I don't see why userspace can't do this themselves. I've seen that
> it makes things easier in SNA/X, but for a general purpose interface
> that argument doesn't cut too much.

I have completely opposite viewpoint: A general purpose interface is not
a page interface, and that this interface trades a small amount of
kernel complexity (tracking the offset_in_page) so that userspace has a
flexible interface that matches its requirements.

> - I'm also a bit afraid that our code implicitly assumes that
> size/offset are always page-aligned and I kinda want to avoid that we
> have to audit for such issues from here on. We've blown up in the past
> assuming that size > 0 already, I think we're set to blow up on this
> one here.

Now that we can distinguish between size and num_pages, there is no
longer a need for size to be page aligned (and is currently redundant).
 
> In any case, if you really want to stick to this I want this to be
> explictly track in an obj->reloc_gtt_offset_adjustment or something

Sure, let's call it obj->gtt_offset:12;

> which is very loudly yelling at people to make sure no one trips over
> it. Tracking the adjustment in a separate field, which would only ever
> be used in the reloc code would address all my concerns (safe for the
> api ugliness one).

And everywhere that deals in GTT addresses.
 
> >> - I915_USERPTR_UNSYNCHRONIZED ... eeek. That means that despite everyone
> >>   having mmu notifiers enabled in their distro config, you make sure sna
> >>   doesn't hit it. Imo not enough testing coverage ;-) Or this there
> >>   another reason behind this than "mmu notifiers are too slow"?
> >>
> >>   Generally I'm a bit sloppy with going root-only for legacy X stuff (like
> >>   scanline waits), but this here looks very much generally useful. So not
> >>   exemption-material imo.
> >
> > Strongly disagree. Most of my machines do not have mmu-notifiers and
> > would still like to benefit from userptr and I see no reason why we need
> > to force mmu-notifiers.
> 
> Note that I didn't shout against the mmu_notifier-less support
> (although I'm honestly not too happy about it), what I don't like is
> the override bit disabling the mmu_notifiers even if we have them.
> Since that will mean that the code won't be tested through SNA, and so
> has a good chance of being buggy. Once mesa comes around and uses it,
> it'll nicely blow up. And one of the reason Jesse is breathing down my
> neck to merge this is "other guys are interested in this at intel".

I don't see how you can reconcile that viewpoint. In order for userspace
to be able to use userptr without mmu-notifiers it has to be very
careful about managing synchronisation of such pointers across vma
events. So userptr should only be allowed to opt-in to such a precarious
arrangement. And because it was so easy for you to shoot yourself and
the entire system in the head by getting such synchronisation wrong, the
suggestion was to only allow root access to the shotgun. Once it is opting
in, behaviour of userptr should then be consistent across all
configurations i.e. it should always avoid the mmu-notifier if userspace
asks.

As for testing using SNA, you can just ask SNA to only use mmu-notifier
backed userptr and so only run when mmu-notifiers are available.
 
> >> - On a quick read I've seen some gtt mmap support remnants. This scares
> >>   me, a roundabout njet! would appease. Though I think that should already
> >>   happen with the checks we have to reject snoopable buffers?
> >
> > That's because there are platforms where it is theorectically possible
> > and whilst I have no use case for it, I wanted to make it work
> > nevertheless. I still think it is possible, but I could not see a way to
> > do so without completely replacing the drm vm code.
> 
> If I understand things correctly we should be able to block this
> simply by refusing to create an mmap offset for a userptr backed
> object.

But I want to be able to support GTT mappings wherever possible. The
basic principle of making sure every bo is first class, so that there
are no surprises. Such as stolen, dmabuf...
 
> >> - I think we should reject set_cacheing requests on usersptr objects.
> >
> > I don't think that is strictly required, just like we should not limit
> > the user from using set_tiling. (Though the user is never actually going
> > to tell the kernel about such tiling...).
> 
> Yeah, I guess we could allow people to shot their foot off. Otoh it
> adds another dimension to the userptr interface, which we need to make
> sure it works. Similar besides set_tiling is probably also
> prime_export.
> 
> The reason I'd prefer to lock things down is that we have fairly nice
> coverage for normal gem objects wrt exercising corner-cases with igt.
> If we don't disallow the same corner-cases with userptr, I'd prefer
> the tests to also cover those ... Disallowing is cheaper ;-)

See above: every bo should be first class...
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre