linux-kernel - Re: [PATCH 6/6] x86: Use clwb in drm_clflush_virt

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Date:	Thu, 13 Nov 2014 19:33:54 +0200
From:	Ville Syrjälä <ville.syrjala@...ux.intel.com>
To:	Borislav Petkov <bp@...en8.de>
Cc:	Andy Lutomirski <luto@...capital.net>,
	intel-gfx@...ts.freedesktop.org, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	DRI <dri-devel@...ts.freedesktop.org>,
	Ingo Molnar <mingo@...nel.org>,
	Ross Zwisler <ross.zwisler@...ux.intel.com>,
	H Peter Anvin <h.peter.anvin@...el.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 6/6] x86: Use clwb in drm_clflush_virt_range

On Thu, Nov 13, 2014 at 06:11:33PM +0100, Borislav Petkov wrote:
> On Thu, Nov 13, 2014 at 08:38:23AM -0800, Andy Lutomirski wrote:
> > On Nov 13, 2014 3:20 AM, "Borislav Petkov" <bp@...en8.de> wrote:
> > >
> > > On Wed, Nov 12, 2014 at 07:14:21PM -0800, Andy Lutomirski wrote:
> > > > On 11/11/2014 10:43 AM, Ross Zwisler wrote:
> > > > > If clwb is available on the system, use it in drm_clflush_virt_range.
> > > > > If clwb is not available, fall back to clflushopt if you can.
> > > > > If clflushopt is not supported, fall all the way back to clflush.
> > > >
> > > > I don't know exactly what drm_clflush_virt_range (and the other
> > > > functions you're modifying similarly) are for, but it seems plausible to
> > > > me that they're used before reads to make sure that non-coherent memory
> > > > sees updated data.  If that's true, then this will break it.
> > >
> > > Why would it break it? The updated cachelines will be in memory and
> > > subsequent reads will be serviced from the cache instead from going to
> > > memory as it is not invalidated as it would be by CLFLUSH.
> > >
> > > /me is puzzled.
> > 
> > Suppose you map some device memory WB, and then the device
> > non-coherently updates.  If you want the CPU to see it, you need
> > clflush or clflushopt.  Some architectures might do this for
> > dma_sync_single_for_cpu with DMA_FROM_DEVICE.
> 
> Ah, you're talking about the other way around - the device does the
> writes. Well, the usage sites are all in i915*, maybe we should ask
> them - it looks to me like this is only the CPU making stuff visible in
> the shared buffer but I don't know that code... intel-gfx CCed although
> dri-devel is already on CC.

We use it both ways in i915. So please don't break it.

-- 
Ville Syrjälä
Intel OTC
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