[PATCH 1/6] xen/arm: Fix evaluation of parameters for SMCCC calls

Contrary to what was claimed in commit 67bcf5eae709 ("xen/arm: Simplify type
handling for SMCCC declarations"), there is an important reason to retain the
intermediate variable.  It is unsafe to have any logic between the assignment
of the register variabes and the asm() block they're used in.

This logically reverts commit 67bcf5eae709 ("xen/arm: Simplify type handling
for SMCCC declarations") while retaining the conversions from commit
7f15d5d13221 ("xen/treewide: More typeof() -> auto conversions").

Adjust __declare_arg_0() to match.  It happens to be safe because it's the
first register expression once all macros are expanded, but it really should
be consistent with the others.

Leave a comment explaining why they must be written like this.

Fixes: 67bcf5eae709 ("xen/arm: Simplify type handling for SMCCC declarations")
Reported-by: Jan Setje-Eilers <[email protected]>
Signed-off-by: Andrew Cooper <[email protected]>
---
CC: Stefano Stabellini <[email protected]>
CC: Julien Grall <[email protected]>
CC: Volodymyr Babchuk <[email protected]>
CC: Bertrand Marquis <[email protected]>
CC: Michal Orzel <[email protected]>
CC: Jan Setje-Eilers <[email protected]>
---
 xen/arch/arm/include/asm/smccc.h | 31 +++++++++++++++++++++++--------
 1 file changed, 23 insertions(+), 8 deletions(-)

diff --git a/xen/arch/arm/include/asm/smccc.h b/xen/arch/arm/include/asm/smccc.h
index 62c6985e7315..53cdddb690b7 100644
--- a/xen/arch/arm/include/asm/smccc.h
+++ b/xen/arch/arm/include/asm/smccc.h
@@ -108,37 +108,52 @@ struct arm_smccc_res {
 #define __constraint_read_6 __constraint_read_5, "r" (arg6)
 #define __constraint_read_7 __constraint_read_6, "r" (arg7)
 
+/*
+ * Macro arguments MUST be evaluated before being assigned to a register
+ * variable.
+ *
+ * This is manual register scheduling for the asm() statement, and any other
+ * logic to evaluate may clobber the already-scheduled registers.
+ */
 #define __declare_arg_0(a0, res)                            \
+    auto __a0 = (uint32_t)(a0);                             \
     struct arm_smccc_res    *___res = (res);                \
-    register unsigned long  arg0 ASM_REG(0) = (uint32_t)(a0)
+    register unsigned long  arg0 ASM_REG(0) = __a0
 
 #define __declare_arg_1(a0, a1, res)                        \
+    auto __a1 = (a1);                                       \
     __declare_arg_0(a0, res);                               \
-    register auto           arg1 ASM_REG(1) = (a1)
+    register auto           arg1 ASM_REG(1) = __a1
 
 #define __declare_arg_2(a0, a1, a2, res)                    \
+    auto __a2 = (a2);                                       \
     __declare_arg_1(a0, a1, res);                           \
-    register auto           arg2 ASM_REG(2) = (a2)
+    register auto           arg2 ASM_REG(2) = __a2
 
 #define __declare_arg_3(a0, a1, a2, a3, res)                \
+    auto __a3 = (a3);                                       \
     __declare_arg_2(a0, a1, a2, res);                       \
-    register auto           arg3 ASM_REG(3) = (a3)
+    register auto           arg3 ASM_REG(3) = __a3
 
 #define __declare_arg_4(a0, a1, a2, a3, a4, res)        \
+    auto __a4 = (a4);                                   \
     __declare_arg_3(a0, a1, a2, a3, res);               \
-    register auto           arg4 ASM_REG(4) = (a4)
+    register auto           arg4 ASM_REG(4) = __a4
 
 #define __declare_arg_5(a0, a1, a2, a3, a4, a5, res)    \
+    auto __a5 = (a5);                                   \
     __declare_arg_4(a0, a1, a2, a3, a4, res);           \
-    register auto           arg5 ASM_REG(5) = (a5)
+    register auto           arg5 ASM_REG(5) = __a5
 
 #define __declare_arg_6(a0, a1, a2, a3, a4, a5, a6, res)    \
+    auto __a6 = (a6);                                       \
     __declare_arg_5(a0, a1, a2, a3, a4, a5, res);           \
-    register auto           arg6 ASM_REG(6) = (a6)
+    register auto           arg6 ASM_REG(6) = __a6
 
 #define __declare_arg_7(a0, a1, a2, a3, a4, a5, a6, a7, res)    \
+    auto __a7 = (a7);                                           \
     __declare_arg_6(a0, a1, a2, a3, a4, a5, a6, res);           \
-    register auto           arg7 ASM_REG(7) = (a7)
+    register auto           arg7 ASM_REG(7) = __a7
 
 #define ___declare_args(count, ...) __declare_arg_ ## count(__VA_ARGS__)
 #define __declare_args(count, ...)  ___declare_args(count, __VA_ARGS__)

base-commit: 79225a0c77e13b693b4d2b903a88289704b79db6
-- 
2.39.5