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

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

On Tue, Jul 02, 2013 at 12:00:48PM -0700, Paul Berry wrote:
>    On 28 June 2013 15:39, Chris Wilson <[1]chris@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.

You do not on the BLT ring, and have not done historically. The EU is no
longer padded. We have been purposely lax in this area.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre
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