Re: [Xen-devel] [PATCH 1/3] xen/vt-d: need barriers to workaround CLFLUSH

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Mon, May 04, 2015 at 06:39:56PM +0800, Chen, Tiejun wrote:
> On 2015/5/4 16:52, Jan Beulich wrote:
> >>>>On 04.05.15 at 04:16, <tiejun.chen@xxxxxxxxx> wrote:
> >>--- a/xen/drivers/passthrough/vtd/x86/vtd.c
> >>+++ b/xen/drivers/passthrough/vtd/x86/vtd.c
> >>@@ -56,7 +56,9 @@ unsigned int get_cache_line_size(void)
> >>
> >>  void cacheline_flush(char * addr)
> >>  {
> >>+    mb();
> >>      clflush(addr);
> >>+    mb();
> >>  }
> >
> >I think the purpose of the flush is to force write back, not to evict
> >the cache line, and if so wmb() would appear to be sufficient. As
> >the SDM says that's not the case, a comment explaining why wmb()
> >is not sufficient would seem necessary. Plus in the description I
> 
> Seems wmb() is not sufficient here.
> 
> "CLFLUSH is only ordered by the MFENCE instruction. It is not guaranteed to
> be ordered by any other fencing, serializing or other CLFLUSH instruction."

That is incorrect. We have observed that CLFLUSH instruction do serialize each
other. That is if on a core you send a bunch of CLFLUSH it stalls the pipeline.

Cc-ing Boris who discovered this.
> 
> Thanks
> Tiejun
> 
> >think "serializing" needs to be changed to "fencing", as serialization
> >is not what we really care about here. If you and the maintainers
> >agree, I could certainly fix up both aspects while committing.
> >
> >Jan
> >
> >
> >
> 
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