[Xen-devel] [PATCH v2] xen: arm: clarify cacheability requirements of hypercall arguments.

[Ian Campbell <ian.campbell@xxxxxxxxxx>]

Accepting hypercall arguments which are either consistently in cached or
uncached is tricky and/or potentially slow, requiring a guest mapping lookup
to determine whether/when to do a cache clean or invalidate.

There are very few reasons, and no current use cases in practice, for a guest
to use uncached memory for their hypercall arguments. Therefore mandate that
all hypercall arguments must be mapped inner-cacheable.

Do not place any restriction on the outer-cacheability or on the cache
fill/flush strategy used.

If use cases arise then we can consider specific exemptions to this rule.

Signed-off-by: Ian Campbell <ian.campbell@xxxxxxxxxx>
---
v2: Expliclty mention Outer-cacheability and cache allocation strategy.
---
 xen/include/public/arch-arm.h |    5 +++++
 1 file changed, 5 insertions(+)

diff --git a/xen/include/public/arch-arm.h b/xen/include/public/arch-arm.h
index 475cb4a..ef6217d 100644
--- a/xen/include/public/arch-arm.h
+++ b/xen/include/public/arch-arm.h
@@ -58,6 +58,11 @@
  * (AAPCS64). Where there is a conflict the 64-bit standard should be
  * used regardless of guest type. Structures which are passed as
  * hypercall arguments are always little endian.
+ *
+ * All hypercall arguments passed via a pointer to guest memory must
+ * reside in memory which is mapped as Normal Inner-cacheable. Any
+ * Inner cache allocation strategy (Write-Back, Write-Through etc) is
+ * acceptable. There is no restriction on the Outer-cacheability.
  */
 
 /*
-- 
1.7.10.4


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