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

zhuqiuer
2024-04-09T11:39:07Z

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