[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

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();
       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."

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?

Thanks
Tiejun

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel


 


Rackspace

Lists.xenproject.org is hosted with RackSpace, monitoring our
servers 24x7x365 and backed by RackSpace's Fanatical Support®.