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

[Ben Widawsky <ben@bwidawsk.net>]

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.
> -Chris
> 
> -- 
> Chris Wilson, Intel Open Source Technology Centre

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.

-- 
Ben Widawsky, Intel Open Source Technology Center