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

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();
+    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."


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.


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