On 28 June 2013 15:39, Chris Wilson <chris at chris-wilson.co.uk> wrote: > On Fri, Jun 28, 2013 at 03:23:31PM -0700, Ben Widawsky wrote: > > This series originated from the request from Paul, "can you enable page > > faults"? After some though and discussion, we came up with 3 debug > features to > > implement: > > The issue lies in that the CS and EU units like to prefetch 128 bytes > and will cross page boundaries. Userspace is rather lax in providing the > extra page (or preventing the read past the end of its bo) and so > without adding a sentinel page behind every bo you quickly generate > false positives. (Unless you also run a fixed userspace). > > If you are prepared to fix userspace, tweaking the kernel not to install > scratch pages everywhere is trivial. > Mesa already adds the necessary padding to EU programs to ensure that prefetch won't cause a page fault, and because of its "stack and heap" model for batch buffers, CS prefetching shouldn't cause a page fault either. I don't know whether the 2D drivers do something similar, so they might potentially need fixing. Having tracked down a number of painful Mesa bugs over the last two years where we supplied the GPU with an incorrect pointer, or we sized a buffer incorrectly causing other buffers to be corrupted, I am fairly certain that being able to turn off scratch pages will be a benefit in debugging--these debug options should turn confusing, sporadic failures (and failures that don't get noticed for months) into easily reproducible failures that get noticed quickly. So these debug flags will definitely be useful in development. If the 2D driver is lax about providing the necessary padding for prefetch, then that will make things more difficult, but the debug flags should still be useful. It just means that when using the debug flags, I'll have to run my piglit tests under gbm. I'll try to give the patches a shot soon to make sure they work. -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.freedesktop.org/archives/intel-gfx/attachments/20130702/08aae993/attachment-0001.html>
