Re: [PATCH 6/6] x86: Use clwb in drm_clflush_virt_range

Andy Lutomirski
2014-11-13T16:38:47Z

On Nov 13, 2014 3:20 AM, "Borislav Petkov" <bp@alien8.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