Received-SPF: pass (google.com: domain of linux-kernel+bounces-136793-linux.lists.archive=gmail.com@vger.kernel.org designates 2604:1380:4601:e00::3 as permitted sender) client-ip=2604:1380:4601:e00::3;
From: zhuqiuer <zhuqiuer1@huawei.com>
To: <linux@armlinux.org.uk>, <nathan@kernel.org>, <ndesaulniers@google.com>,
	<morbo@google.com>, <justinstitt@google.com>
CC: <linux-arm-kernel@lists.infradead.org>, <linux-kernel@vger.kernel.org>
Subject: [PATCH] ARM: Add a memory clobber to the fmrx instruction
Date: Tue, 9 Apr 2024 19:38:44 +0800
Message-ID: <20240409113844.4813-1-zhuqiuer1@huawei.com>
Precedence: bulk
MIME-Version: 1.0
Content-Type: text/plain

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 | 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