Re: [PATCH 18/18] drm/i915: Introduce mapping of user pages into video memory (userptr) ioctl

From: Eric Anholt <eric@anholt.net>
To: Daniel Vetter <daniel@ffwll.ch>, Jesse Barnes <jbarnes@virtuousgeek.org>
Cc: intel-gfx@lists.freedesktop.org
Subject: Re: [PATCH 18/18] drm/i915: Introduce mapping of user pages into video memory (userptr) ioctl
Date: Tue, 23 Oct 2012 10:50:10 -0700	[thread overview]
Message-ID: <87vce16pt9.fsf@eliezer.anholt.net> (raw)
In-Reply-To: <CAKMK7uH7i9oTbycDkUd6jcsdJsvkezOpf6CC3FhJfard=L_AyQ@mail.gmail.com>

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Daniel Vetter <daniel@ffwll.ch> writes:

> On Tue, Oct 23, 2012 at 12:21 AM, Jesse Barnes <jbarnes@virtuousgeek.org> wrote:
>> On Fri, 19 Oct 2012 18:03:24 +0100
>> Chris Wilson <chris@chris-wilson.co.uk> 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).
>>>
>>> v2: Compile with CONFIG_MMU_NOTIFIER
>>
>> I want to understand the root-only nature of this better.
>>
>> Is locking complexity the only reason we can't treat these pages like
>> normal BOs which can be swapped out from under us as long as they're
>> not pinned into the GTT?  Or are there other complications in managing
>> the refcount for these pages?
>
> Essentially the complication in pinning tons of memory that we don't
> control, and hence might upset other kernel mm parts (like page
> migration). For native objects we can always fix this by intercepting
> more of the vm callbacks for the shmem backing storage (i.e. gemfs).
> But for foreing storage we need to use the mmu_notifiers, and atm
> those expect that a pte shootdown is synchronous (which we can't do),
> so there's some work left to get done imo.

So, the code isn't done, and the solution is to push it as root-only?
Seriously?

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