[PATCH] xen/arm: un-break build with clang

[Stewart Hildebrand <stewart.hildebrand@xxxxxxx>]

clang doesn't like extern with __attribute__((__used__)):

  ./arch/arm/include/asm/setup.h:171:8: error: 'used' attribute ignored 
[-Werror,-Wignored-attributes]
  extern DEFINE_BOOT_PAGE_TABLE(boot_pgtable);
         ^
  ./arch/arm/include/asm/lpae.h:273:29: note: expanded from macro 
'DEFINE_BOOT_PAGE_TABLE'
  lpae_t __aligned(PAGE_SIZE) __section(".data.page_aligned")                   
\
                              ^
  ./include/xen/compiler.h:71:27: note: expanded from macro '__section'
  #define __section(s)      __used __attribute__((__section__(s)))
                            ^
  ./include/xen/compiler.h:104:39: note: expanded from macro '__used'
  #define __used         __attribute__((__used__))
                                        ^

Introduce a new EXTERN_DEFINE_BOOT_PAGE_TABLE macro without
__attribute__((__used__)) and use this for the relevant declarations.

Fixes: 1c78d76b67e1 ("xen/arm64: mm: Introduce helpers to 
prepare/enable/disable the identity mapping")
Signed-off-by: Stewart Hildebrand <stewart.hildebrand@xxxxxxx>
---
I tested with clang 12 and clang 16

Here is my make command line:
make -j $(nproc) \
    clang=y \
    CC="clang --target=aarch64-none-linux-gnu -march=armv8a+nocrypto" \
    CXX="clang++ --target=aarch64-none-linux-gnu -march=armv8a+nocrypto" \
    HOSTCC=clang \
    HOSTCXX=clang++ \
    XEN_TARGET_ARCH=arm64 \
    CROSS_COMPILE=aarch64-none-linux-gnu- \
    dist-xen
---
 xen/arch/arm/include/asm/lpae.h  | 4 ++++
 xen/arch/arm/include/asm/setup.h | 8 ++++----
 xen/include/xen/compiler.h       | 1 +
 3 files changed, 9 insertions(+), 4 deletions(-)

diff --git a/xen/arch/arm/include/asm/lpae.h b/xen/arch/arm/include/asm/lpae.h
index 3fdd5d0de28e..294a8aa4bd30 100644
--- a/xen/arch/arm/include/asm/lpae.h
+++ b/xen/arch/arm/include/asm/lpae.h
@@ -273,6 +273,10 @@ lpae_t mfn_to_xen_entry(mfn_t mfn, unsigned int attr);
 lpae_t __aligned(PAGE_SIZE) __section(".data.page_aligned")                   \
     name[XEN_PT_LPAE_ENTRIES]
 
+#define EXTERN_DEFINE_BOOT_PAGE_TABLE(name)                                   \
+extern lpae_t __aligned(PAGE_SIZE) __no_used_section(".data.page_aligned")    \
+    name[XEN_PT_LPAE_ENTRIES]
+
 #define DEFINE_PAGE_TABLES(name, nr)                    \
 lpae_t __aligned(PAGE_SIZE) name[XEN_PT_LPAE_ENTRIES * (nr)]
 
diff --git a/xen/arch/arm/include/asm/setup.h b/xen/arch/arm/include/asm/setup.h
index 38e2ce255fcf..af53e58a6a07 100644
--- a/xen/arch/arm/include/asm/setup.h
+++ b/xen/arch/arm/include/asm/setup.h
@@ -168,13 +168,13 @@ u32 device_tree_get_u32(const void *fdt, int node,
 int map_range_to_domain(const struct dt_device_node *dev,
                         u64 addr, u64 len, void *data);
 
-extern DEFINE_BOOT_PAGE_TABLE(boot_pgtable);
+EXTERN_DEFINE_BOOT_PAGE_TABLE(boot_pgtable);
 
 #ifdef CONFIG_ARM_64
-extern DEFINE_BOOT_PAGE_TABLE(boot_first_id);
+EXTERN_DEFINE_BOOT_PAGE_TABLE(boot_first_id);
 #endif
-extern DEFINE_BOOT_PAGE_TABLE(boot_second_id);
-extern DEFINE_BOOT_PAGE_TABLE(boot_third_id);
+EXTERN_DEFINE_BOOT_PAGE_TABLE(boot_second_id);
+EXTERN_DEFINE_BOOT_PAGE_TABLE(boot_third_id);
 
 /* Find where Xen will be residing at runtime and return a PT entry */
 lpae_t pte_of_xenaddr(vaddr_t);
diff --git a/xen/include/xen/compiler.h b/xen/include/xen/compiler.h
index 7d7ae2e5e4d9..70ba563e29c2 100644
--- a/xen/include/xen/compiler.h
+++ b/xen/include/xen/compiler.h
@@ -73,6 +73,7 @@
 #define __section(s)      __attribute__((__section__(s)))
 #endif
 #define __used_section(s) __used __attribute__((__section__(s)))
+#define __no_used_section(s) __attribute__((__section__(s)))
 #define __text_section(s) __attribute__((__section__(s)))
 
 #define __aligned(a) __attribute__((__aligned__(a)))
-- 
2.40.1