Re: [PATCH v2] ARM: Add a memory clobber to the fmrx instruction

Ard Biesheuvel
2024-04-10T12:31:46Z

On Wed, 10 Apr 2024 at 04:41, zhuqiuer <zhuqiuer1@huawei.com> wrote:
>
> 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>

This also fixes the issue I observed so

Reviewed-by: Ard Biesheuvel <ardb@kernel.org>

> ---
> 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