Messages in this thread | | | From | Ard Biesheuvel <> | Date | Wed, 10 Apr 2024 14:31:11 +0200 | Subject | Re: [PATCH v2] ARM: Add a memory clobber to the fmrx instruction |
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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 >
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