Re: [PATCH 0/6] Page faults to help user space debug

[Chris Wilson <chris@chris-wilson.co.uk>]

On Fri, Jun 28, 2013 at 04:30:44PM -0700, Ben Widawsky wrote:
> On Fri, Jun 28, 2013 at 11:39:53PM +0100, Chris Wilson 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.
> 
> I'll leave the further discussion of whether or not they still want all
> the features to Paul (I think the synchronous unbinding execbuf is
> useful regardless).
> 
> As for the trivial, "remove the scratch page" I didn't think that was
> the plan as generally we'd like to carry on as long as possible, and we
> know faults are fatal. I think keeping the scratch page, even if we
> didn't have to worry about prefetching is what we want.

No plans to remove the scratch page as it hides too many bugs, which
generally are fatal enough when run for long enough. I was just offering
up my experience on using page faults.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre