[Mesa-dev] [PATCH] mesa: Add missing _mm_mfence() before streaming loads.

[Nuzman, Joseph]

The SDM reference is admittedly a little fuzzy on the required fencing, but an SFENCE has no impact on the streaming loads.  MFENCE is required to enforce the ordering of the streaming loads.

Joe

-----Original Message-----
From: Chris Wilson [mailto:chris at chris-wilson.co.uk] 
Sent: Tuesday, October 06, 2015 12:13
To: Justen, Jordan L
Cc: Matt Turner; mesa-dev at lists.freedesktop.org; Nuzman, Joseph
Subject: Re: [Mesa-dev] [PATCH] mesa: Add missing _mm_mfence() before streaming loads.

On Mon, Oct 05, 2015 at 12:01:25PM -0700, Jordan Justen wrote:
> On 2015-10-05 11:55:57, Matt Turner wrote:
> > On Thu, Oct 1, 2015 at 11:20 AM, Jordan Justen 
> > <jordan.l.justen at intel.com> wrote:
> > > On 2015-10-01 10:11:33, Matt Turner wrote:
> > >> According to the Intel Software Development Manual:
> > >
> > > How about a more specific doc location?
> > >
> > > According to the Intel Software Development Manual (Volume 1: 
> > > Basic Architecture, 12.10.3 Streaming Load Hint Instruction):
> > 
> > Sure. Thanks for looking that up for me.
> > 
> > >>    Streaming loads may be weakly ordered and may appear to software to
> > >>    execute out of order with respect to other memory operations.
> > >>    Software must explicitly use fences (e.g. MFENCE) if it needs to
> > >>    preserve order among streaming loads or between streaming loads and
> > >>    other memory operations.
> > >
> > > Does this mean we need a mfence following the load as well?
> > 
> > I didn't think so, and Joseph has confirmed.
> > 
> > Can I consider this a Reviewed-by?
> 
> Reviewed-by: Jordan Justen <jordan.l.justen at intel.com>

I believe you only need a SFENCE here not a full MFENCE.
-Chris

--
Chris Wilson, Intel Open Source Technology Centre
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