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Re: [Xen-devel] [PATCH] ARM: cache coherence problem in guestcopy.c

On Tue, 2013-06-18 at 13:18 +0100, Ian Campbell wrote:
> On Tue, 2013-06-18 at 12:05 +0000, Jaeyong Yoo wrote:
> > > We were also speculating that we probably want some DMBs in
> > > context_switch_{from,to} as well as at return_to_guest.
> > 
> > Actually, I just learned ReOrder Buffer, and it looks like so.

Does this patch help with the issue you are seeing?

I think in theory it should *BUT* (and it's a big But)...

... it doesn't work for me on an issue I've been seeing booting dom0 on
the Foundation model. However doing the dmb in map_domain_page() *twice*
does work for me. In fact doing a dmb+nop works too. This is
inexplicable to me. It may be a model bug or it may (more likely) be
indicative of some other underlying issue. I also tried dsb() instead of
dmb() and that doesn't make a difference.

Anyway, I'm interested about its behaviour on real hardware.

These are doing full system barriers. I think in reality only a "dmb
nsh" should be needed for this use case, since we only care about the
local processor. I didn't want to go there when the supposedly more
obvious case wasn't doing as I expected!


diff --git a/xen/arch/arm/mm.c b/xen/arch/arm/mm.c
index beae61e..ef67953 100644
--- a/xen/arch/arm/mm.c
+++ b/xen/arch/arm/mm.c
@@ -218,6 +218,13 @@ void *map_domain_page(unsigned long mfn)
+    /*
+     * Ensure that any previous writes which occurred via another
+     * mapping, specifically the guest's own mapping have been
+     * completed such that they are visible via this mapping.
+     */
+    dmb();
     /* The map is laid out as an open-addressed hash table where each
      * entry is a 2MB superpage pte.  We use the available bits of each
      * PTE as a reference count; when the refcount is zero the slot can
@@ -286,6 +297,13 @@ void unmap_domain_page(const void *va)
+    /*
+     * Ensure that any writes which occurred via this mapping have been
+     * completed such that they are visible via other virtual
+     * mappings, specifically the guest's own mapping.
+     */
+    dmb();

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