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riscv: Fix build with CONFIG_CC_OPTIMIZE_FOR_SIZE=y

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commit 8eb060e101 ("arch/riscv: add Zihintpause support") broke
building with CONFIG_CC_OPTIMIZE_FOR_SIZE enabled (gcc 11.1.0):

  CC      arch/riscv/kernel/vdso/vgettimeofday.o
In file included from <command-line>:
./arch/riscv/include/asm/jump_label.h: In function 'cpu_relax':
././include/linux/compiler_types.h:285:33: warning: 'asm' operand 0 probably does not match constraints
  285 | #define asm_volatile_goto(x...) asm goto(x)
      |                                 ^~~
./arch/riscv/include/asm/jump_label.h:41:9: note: in expansion of macro 'asm_volatile_goto'
   41 |         asm_volatile_goto(
      |         ^~~~~~~~~~~~~~~~~
././include/linux/compiler_types.h:285:33: error: impossible constraint in 'asm'
  285 | #define asm_volatile_goto(x...) asm goto(x)
      |                                 ^~~
./arch/riscv/include/asm/jump_label.h:41:9: note: in expansion of macro 'asm_volatile_goto'
   41 |         asm_volatile_goto(
      |         ^~~~~~~~~~~~~~~~~
make[1]: *** [scripts/Makefile.build:249: arch/riscv/kernel/vdso/vgettimeofday.o] Error 1
make: *** [arch/riscv/Makefile:128: vdso_prepare] Error 2

Having a static branch in cpu_relax() is problematic because that
function is widely inlined, including in some quite complex functions
like in the VDSO. A quick measurement shows this static branch is
responsible by itself for around 40% of the jump table.

Drop the static branch, which ends up being the same number of
instructions anyway. If Zihintpause is supported, we trade the nop from
the static branch for a div. If Zihintpause is unsupported, we trade the
jump from the static branch for (what gets interpreted as) a nop.

Fixes: 8eb060e101 ("arch/riscv: add Zihintpause support")
Signed-off-by: Samuel Holland <samuel@sholland.org>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Cc: stable@vger.kernel.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>
This commit is contained in:
Samuel Holland 2022-09-22 01:09:58 -05:00 committed by Palmer Dabbelt
parent 8eb060e101
commit 0b1d60d6dd
No known key found for this signature in database
GPG Key ID: 2E1319F35FBB1889
2 changed files with 10 additions and 18 deletions
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3
arch/riscv/include/asm/hwcap.h
View File

@ -67,7 +67,6 @@ enum riscv_isa_ext_id {
*/
enum riscv_isa_ext_key {
RISCV_ISA_EXT_KEY_FPU, /* For 'F' and 'D' */
RISCV_ISA_EXT_KEY_ZIHINTPAUSE,
RISCV_ISA_EXT_KEY_MAX,
};
@ -87,8 +86,6 @@ static __always_inline int riscv_isa_ext2key(int num)
return RISCV_ISA_EXT_KEY_FPU;
case RISCV_ISA_EXT_d:
return RISCV_ISA_EXT_KEY_FPU;
case RISCV_ISA_EXT_ZIHINTPAUSE:
return RISCV_ISA_EXT_KEY_ZIHINTPAUSE;
default:
return -EINVAL;
}

25
arch/riscv/include/asm/vdso/processor.h
View File

@ -4,30 +4,25 @@
#ifndef __ASSEMBLY__
#include <linux/jump_label.h>
#include <asm/barrier.h>
#include <asm/hwcap.h>
static inline void cpu_relax(void)
{
if (!static_branch_likely(&riscv_isa_ext_keys[RISCV_ISA_EXT_KEY_ZIHINTPAUSE])) {
#ifdef __riscv_muldiv
int dummy;
/* In lieu of a halt instruction, induce a long-latency stall. */
__asm__ __volatile__ ("div %0, %0, zero" : "=r" (dummy));
int dummy;
/* In lieu of a halt instruction, induce a long-latency stall. */
__asm__ __volatile__ ("div %0, %0, zero" : "=r" (dummy));
#endif
} else {
/*
* Reduce instruction retirement.
* This assumes the PC changes.
*/
/*
* Reduce instruction retirement.
* This assumes the PC changes.
*/
#ifdef __riscv_zihintpause
__asm__ __volatile__ ("pause");
__asm__ __volatile__ ("pause");
#else
/* Encoding of the pause instruction */
__asm__ __volatile__ (".4byte 0x100000F");
/* Encoding of the pause instruction */
__asm__ __volatile__ (".4byte 0x100000F");
#endif
}
barrier();
}