[Xen-devel] [PATCH v8 02/17] Add cmpxchg16b support for x86-64

[Feng Wu <feng.wu@xxxxxxxxx>]

This patch adds cmpxchg16b support for x86-64, so software
can perform 128-bit atomic write/read.

CC: Keir Fraser <keir@xxxxxxx>
CC: Jan Beulich <jbeulich@xxxxxxxx>
CC: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Signed-off-by: Feng Wu <feng.wu@xxxxxxxxx>
---
v8:
- Remove pointless cast when assigning 'new_low'
- properly parenthesize cmpxchg16b()

v7:
- Make the last two parameters of __cmpxchg16b() const
- Remove memory clobber
- Add run-time and build-build check in cmpxchg16b()
- Cast the last two parameter to void * when calling __cmpxchg16b()

v6:
- Fix a typo

v5:
- Change back the parameters of __cmpxchg16b() to __uint128_t *
- Remove pointless cast for 'ptr'
- Remove pointless parentheses
- Use A constraint for the output

v4:
- Use pointer as the parameter of __cmpxchg16b().
- Use gcc's __uint128_t built-in type
- Make the parameters of __cmpxchg16b() void *

v3:
- Newly added.

 xen/include/asm-x86/x86_64/system.h | 33 +++++++++++++++++++++++++++++++++
 1 file changed, 33 insertions(+)

diff --git a/xen/include/asm-x86/x86_64/system.h 
b/xen/include/asm-x86/x86_64/system.h
index 662813a..8f87d1e 100644
--- a/xen/include/asm-x86/x86_64/system.h
+++ b/xen/include/asm-x86/x86_64/system.h
@@ -6,6 +6,39 @@
                                    (unsigned long)(n),sizeof(*(ptr))))
 
 /*
+ * Atomic 16 bytes compare and exchange.  Compare OLD with MEM, if
+ * identical, store NEW in MEM.  Return the initial value in MEM.
+ * Success is indicated by comparing RETURN with OLD.
+ *
+ * This function can only be called when cpu_has_cx16 is true.
+ */
+
+static always_inline __uint128_t __cmpxchg16b(
+    volatile void *ptr, const __uint128_t *old, const __uint128_t *new)
+{
+    __uint128_t prev;
+    uint64_t new_high = *new >> 64;
+    uint64_t new_low = *new;
+
+    ASSERT(cpu_has_cx16);
+
+    asm volatile ( "lock; cmpxchg16b %3"
+                   : "=A" (prev)
+                   : "c" (new_high), "b" (new_low),
+                     "m" (*__xg(ptr)), "0" (*old) );
+
+    return prev;
+}
+
+#define cmpxchg16b(ptr, o, n) ({                           \
+    volatile void *_p = (ptr);                             \
+    ASSERT(!((unsigned long)_p & 0xf));                    \
+    BUILD_BUG_ON(sizeof(*(o)) != sizeof(__uint128_t));     \
+    BUILD_BUG_ON(sizeof(*(n)) != sizeof(__uint128_t));     \
+    __cmpxchg16b(_p, (void *)(o), (void *)(n));            \
+})
+
+/*
  * This function causes value _o to be changed to _n at location _p.
  * If this access causes a fault then we return 1, otherwise we return 0.
  * If no fault occurs then _o is updated to the value we saw at _p. If this
-- 
2.1.0


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