linux-kernel - [PATCH] ARM: Add a memory clobber to the fmrx instruction

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Date: Tue, 9 Apr 2024 19:38:44 +0800
From: zhuqiuer <zhuqiuer1@...wei.com>
To: <linux@...linux.org.uk>, <nathan@...nel.org>, <ndesaulniers@...gle.com>,
	<morbo@...gle.com>, <justinstitt@...gle.com>
CC: <linux-arm-kernel@...ts.infradead.org>, <linux-kernel@...r.kernel.org>
Subject: [PATCH] ARM: Add a memory clobber to the fmrx instruction

The instruction fmrx is used throughout the kernel,
where it is sometimes expected to be skipped
by incrementing the program counter, such as in vfpmodule.c:vfp_init().
Therefore, the instruction should not be reordered when it is not intended.
Adding a barrier() instruction before and after this call cannot prevent
reordering by the compiler, as the fmrx instruction is constrained
by '=r', meaning it works on the general register but not on memory.
To ensure the order of the instruction after compiling,
adding a memory clobber is necessary.

Below is the code snippet disassembled from the method:
vfpmodule.c:vfp_init(), compiled by LLVM.

Before the patching:
xxxxx:   xxxxx    bl  c010c688 <register_undef_hook>
xxxxx:   xxxxx    mov r0, r4
xxxxx:   xxxxx    bl  c010c6e4 <unregister_undef_hook>
..
xxxxx:   xxxxx    bl  c0791c8c <printk>
xxxxx:   xxxxx    movw    r5, #23132  ; 0x5a5c
xxxxx:   xxxxx    vmrs    r4, fpsid  <- this is the fmrx instruction

After the patching:
xxxxx:   xxxxx    bl  c010c688 <register_undef_hook>
xxxxx:   xxxxx    mov r0, r4
xxxxx:   xxxxx    vmrs    r5, fpsid  <- this is the fmrx instruction
xxxxx:   xxxxx    bl  c010c6e4 <unregister_undef_hook>

Signed-off-by: zhuqiuer <zhuqiuer1@...wei.com>
---
 arch/arm/vfp/vfpinstr.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/arm/vfp/vfpinstr.h b/arch/arm/vfp/vfpinstr.h
index 3c7938fd40aa..e70129e10b8e 100644
--- a/arch/arm/vfp/vfpinstr.h
+++ b/arch/arm/vfp/vfpinstr.h
@@ -68,7 +68,7 @@
 	u32 __v;			\
 	asm(".fpu	vfpv2\n"	\
 	    "vmrs	%0, " #_vfp_	\
-	    : "=r" (__v) : : "cc");	\
+	    : "=r" (__v) : : "memory", "cc");	\
 	__v;				\
  })

-- 
2.12.3