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

[Chris Wilson]

On Wed, Jan 29, 2014 at 10:58:48PM +0100, Daniel Vetter wrote:
> On Wed, Jan 29, 2014 at 10:53 PM, Chris Wilson <chris at chris-wilson.co.uk> wrote:
> > On Wed, Jan 29, 2014 at 09:25:51PM +0100, Daniel Vetter wrote:
> >> So originally I've thought we need this due to the massive overhead of the
> >> mmu notifier. But now with the nice shared mmu notifiers I've thought that
> >> overhead is gone I prefer to also ditch this option.
> >>
> >> Same goes about the MMU_NOTIFIER conditional code, imo we simply should
> >> select this - most distros will have it anyway and users will be really
> >> suprised if they lose userspace driver features for seemingly irrelevant
> >> reasons.
> >
> > Seriously? You think the overhead is magically gone?
> 
> Well the once-per-process overhead is still there, and imo it's ok to
> eat that. But the complaints I've heard concerned the per-object
> overhead, so I wonder how much of that is still relevant.

I am still annoyed by the thought of having to enable an extra feature
in my kernels, and the extra code that is then run on every mm
operation. (Mixing mmu_notifiers + mm debuging was an especially
unpleasant experience that I don't wish to ever do again.)

Numbers talk though, if we can't demonstrate a significant difference
between the two, it can die. Keeping a debug mode to turn off
mmu_notifiers would still be good so that we can keep track of any
impact over time.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre