[Xen-devel] [PATCH] xen/arm: Tighten memory attribute requirement for memory shared

[Julien Grall <julien.grall@xxxxxxx>]

Xen allows shared mapping to be Normal inner-cacheable with any inner cache
allocation strategy and no restriction of the outer-cacheability.

However, Xen is always mapping those region Normal Inner Write-Back
Outer Write-Back Inner-shareable. Per B2.8 "Mismatched memory
attributes" in ARM DDI 0487B.a, if the guest is not using the exact same
memory attributes (excluding any cache allocation hints) for the shared
region then the region will be accessed with mismatched attributes.

This will result to potential loss of coherency, and may impact the
performance.

Given that the ARM ARM strongly recommends to avoid using mismatched
attributes, we should impose shared region to be Normal Inner Write-Back
Outer Write-Back Inner-shareable.

Signed-off-by: Julien Grall <julien.grall@xxxxxxx>
---
 xen/include/public/arch-arm.h | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/xen/include/public/arch-arm.h b/xen/include/public/arch-arm.h
index bd974fb13d..8f9d06ef7f 100644
--- a/xen/include/public/arch-arm.h
+++ b/xen/include/public/arch-arm.h
@@ -61,15 +61,15 @@
  *
  * All memory which is shared with other entities in the system
  * (including the hypervisor and other guests) must reside in memory
- * which is mapped as Normal Inner-cacheable. This applies to:
+ * which is mapped as Normal Inner Write-Back Outer Write-Back Inner-Shareable.
+ * This applies to:
  *  - hypercall arguments passed via a pointer to guest memory.
  *  - memory shared via the grant table mechanism (including PV I/O
  *    rings etc).
  *  - memory shared with the hypervisor (struct shared_info, struct
  *    vcpu_info, the grant table, etc).
  *
- * Any Inner cache allocation strategy (Write-Back, Write-Through etc)
- * is acceptable. There is no restriction on the Outer-cacheability.
+ * Any cache allocation hints are acceptable.
  */
 
 /*
-- 
2.11.0


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