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

On 2015/5/4 18:52, Tian, Kevin wrote:
From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
Sent: Monday, May 04, 2015 6:44 PM

On 04.05.15 at 12:39, <tiejun.chen@xxxxxxxxx> 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();
+    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

Right - that's what I said in the second sentence.

btw why do we need two fences here? Suppose we just care about
writes before the flush point...

The first MFENCE guarantees all load/store visible before flush cache line. But the second MFENCE just makes sure CLFLUSH is not ordered by that ensuing load/store, right?


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