[Intel-gfx] [PATCH 1/6] drm/i915: Eliminate vmap overhead for cmd parser

[Chris Wilson]

On Thu, Oct 01, 2015 at 03:37:21PM +0300, Ville Syrjälä wrote:
> On Thu, Oct 01, 2015 at 12:57:10PM +0100, Chris Wilson wrote:
> > -	while (cmd < batch_end) {
> > -		const struct drm_i915_cmd_descriptor *desc;
> > -		u32 length;
> > +			k = i;
> > +			if (k > PAGE_SIZE - out)
> > +				k = PAGE_SIZE - out;
> > +			if (k == PAGE_SIZE)
> > +				copy_page(dst, src);
> > +			else
> > +				memcpy(dst + out, src + j, k);
> > +
> > +			out += k;
> > +			j += k;
> > +			i -= k;
> > +		} while (i);
> > +
> > +		cmd = src + in;
> 
> So you're now checking the src batch? What prevents userspace from
> overwriting it with eg. NOPS between the time you copied it and the
> time you check it?

Zilch. I picked src as it was already in the CPU cache, whereas dst will
be WC later. To satisfy you and byt, I need to stage the copy into a
temporary page, scan it, then copy into dst.

The silver lining is that it does remove some lines of code. I'm pretty
confident that the double copy should not be noticed if I remember about
the cache-trashing of kunmap and carefully manage that.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre