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

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Date:	Thu, 13 Nov 2014 08:38:23 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Borislav Petkov <bp@...en8.de>
Cc:	DRI <dri-devel@...ts.freedesktop.org>,
	Ross Zwisler <ross.zwisler@...ux.intel.com>,
	X86 ML <x86@...nel.org>, Thomas Gleixner <tglx@...utronix.de>,
	David Airlie <airlied@...ux.ie>,
	H Peter Anvin <h.peter.anvin@...el.com>,
	Ingo Molnar <mingo@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 6/6] x86: Use clwb in drm_clflush_virt_range

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.

I'm not sure that such a thing exists on x86.

--Andy
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