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Date:   Fri, 28 Feb 2020 10:10:41 -0600
From:   Josh Poimboeuf <jpoimboe@redhat.com>
To:     Chris Wilson <chris@chris-wilson.co.uk>
Cc:     linux-kernel@vger.kernel.org, Randy Dunlap <rdunlap@infradead.org>,
        Peter Zijlstra <peterz@infradead.org>,
        intel-gfx@lists.freedesktop.org
Subject: Re: [PATCH] drm/i915: Minimize uaccess exposure in
 i915_gem_execbuffer2_ioctl()
Message-ID: <20200228161041.ov7d5ox7myrnr4gi@treble>
References: <ed52cfb852d2772bf20f48614d75f1d1b1451995.1582841072.git.jpoimboe@redhat.com>
 <158284236096.19174.6917853940060252533@skylake-alporthouse-com>
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On Thu, Feb 27, 2020 at 10:26:00PM +0000, Chris Wilson wrote:
> > --- a/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
> > +++ b/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
> > @@ -2947,6 +2947,13 @@ i915_gem_execbuffer2_ioctl(struct drm_device *dev, void *data,
> >                         u64_to_user_ptr(args->buffers_ptr);
> >                 unsigned int i;
> >  
> > +               /*
> > +                * Do the call to gen8_canonical_addr() outside the
> > +                * uaccess-enabled region to minimize uaccess exposure.
> > +                */
> > +               for (i = 0; i < args->buffer_count; i++)
> > +                       exec2_list[i].offset = gen8_canonical_addr(exec2_list[i].offset);
> 
> 
> Another loop over all the objects, where we intentionally try and skip
> unmodified entries? To save 2 instructions from inside the second loop?
> 
> Colour me skeptical.

So are you're saying these arrays can be large and that you have
performance concerns?

-- 
Josh