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From: zhuqiuer <zhuqiuer1@huawei.com>
To: <nathan@kernel.org>
CC: <ardb@kernel.org>, <justinstitt@google.com>,
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Subject: [PATCH v2] ARM: Add a memory clobber to the fmrx instruction
Date: Wed, 10 Apr 2024 10:41:26 +0800
Message-ID: <20240410024126.21589-2-zhuqiuer1@huawei.com>
In-Reply-To: <20240410024126.21589-1-zhuqiuer1@huawei.com>
References: <20240409164641.GC3219862@dev-arch.thelio-3990X>
 <20240410024126.21589-1-zhuqiuer1@huawei.com>
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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@huawei.com>
---
 arch/arm/vfp/vfpinstr.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/arm/vfp/vfpinstr.h b/arch/arm/vfp/vfpinstr.h
index 3c7938fd40aa..ae2c9b9b7701 100644
--- a/arch/arm/vfp/vfpinstr.h
+++ b/arch/arm/vfp/vfpinstr.h
@@ -68,14 +68,14 @@
 	u32 __v;			\
 	asm(".fpu	vfpv2\n"	\
 	    "vmrs	%0, " #_vfp_	\
-	    : "=r" (__v) : : "cc");	\
+	    : "=r" (__v) : : "memory", "cc");	\
 	__v;				\
  })
 
 #define fmxr(_vfp_,_var_)		\
 	asm(".fpu	vfpv2\n"	\
 	    "vmsr	" #_vfp_ ", %0"	\
-	   : : "r" (_var_) : "cc")
+	   : : "r" (_var_) : "memory", "cc")
 
 #else
 
-- 
2.12.3