[PATCH] x86/mm: fix objtool failure with KMSAN enabled

[Dmitry Voytik <voytikd@gmail.com>]

This patch fixes broken builds with defconfig + CONFIG_KMSAN +
CONFIG_DEBUG_INFO_*.

To reproduce the issue before the fix:
  make mrproper
  make LLVM=1 defconfig
  ./scripts/config -e CONFIG_KMSAN \
    -e CONFIG_DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT
  make LLVM=1 olddefconfig
  make LLVM=1 -j(nproc) vmlinux
  ...
    LD      vmlinux.o
  vmlinux.o: warning: objtool: folio_zero_user+0x801: undefined stack state
  vmlinux.o: error: objtool: folio_zero_user+0x801: unknown CFA base reg -1
  make[2]: *** [scripts/Makefile.vmlinux_o:76: vmlinux.o] Error 255

objtool in verbose mode shows how the frame pointer is omitted:
  make LLVM=1 OBJTOOL_VERBOSE=1 -j(nproc) vmlinux
  ...
  b15a2c:  folio_zero_user+0x7fc      xor    %eax,%eax
  b15a2e:  folio_zero_user+0x7fe      mov    %rcx,%rsp
  b15a31:  folio_zero_user+0x801      mov    %r14,%rdi
  b15a34:  folio_zero_user+0x804      mov    %rbx,%rcx
  b15a37:  folio_zero_user+0x807      call   0xb15a3c <__clear_pages_unrol

After the fix, the frame pointer is back:
  b15a37: 31 c0                       xor    %eax,%eax
  b15a39: 48 89 ec                    mov    %rbp,%rsp
  b15a3c: 4c 89 f7                    mov    %r14,%rdi
  b15a3f: 48 89 d9                    mov    %rbx,%rcx
  b15a42: e8 00 00 00 00              call   b15a47 <folio_zero_user+0x817>

It seems the issue was introduced by
commit 54a6b89a3db2 ("x86/mm: simplify clear_page_*")

The actual fix is to revert the change how ASM_CALL_CONSTRAINT is
positioned.
Additionally, reintroduce asm_inline to prevent potential compiler
rejection of inlining.

Link: https://lore.kernel.org/CAAX90H2_RPnZL_dFYN7cQF6yt-wAweKKSx2=6e2aZ0kv+Pm+NQ@mail.gmail.com
Signed-off-by: Dmitry Voytik <voytikd@gmail.com>
---
 arch/x86/include/asm/page_64.h | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/arch/x86/include/asm/page_64.h b/arch/x86/include/asm/page_64.h
index 1895c207f629..de8604f0a3a1 100644
--- a/arch/x86/include/asm/page_64.h
+++ b/arch/x86/include/asm/page_64.h
@@ -100,12 +100,13 @@ static inline void clear_pages(void *addr, unsigned int npages)
 	 * __clear_pages_unrolled() are part of the inline asm register
 	 * specification.
 	 */
-	asm volatile(ALTERNATIVE_2("call __clear_pages_unrolled",
-				   "shrq $3, %%rcx; rep stosq", X86_FEATURE_REP_GOOD,
-				   "rep stosb", X86_FEATURE_ERMS)
-			: "+c" (len), "+D" (addr), ASM_CALL_CONSTRAINT
-			: "a" (0)
-			: "cc", "memory");
+	asm_inline volatile(
+		ALTERNATIVE_2("call __clear_pages_unrolled",
+			      "shrq $3, %%rcx; rep stosq", X86_FEATURE_REP_GOOD,
+			      "rep stosb", X86_FEATURE_ERMS)
+		: ASM_CALL_CONSTRAINT, "+c"(len), "+D"(addr)
+		: "a"(0)
+		: "cc", "memory");
 }
 #define clear_pages clear_pages
 
-- 
2.54.0