Re: [PATCH v4] arch/riscv: add Zihintpause support

[Conor.Dooley]

On 11/08/2022 16:17, Palmer Dabbelt wrote:
> EXTERNAL EMAIL: Do not click links or open attachments unless you know the content is safe
> 
> On Mon, 20 Jun 2022 13:15:25 PDT (-0700), daolu@xxxxxxxxxxxx wrote:
>> Implement support for the ZiHintPause extension.
>>
>> The PAUSE instruction is a HINT that indicates the current hart’s rate
>> of instruction retirement should be temporarily reduced or paused.
>>
>> Reviewed-by: Heiko Stuebner <heiko@xxxxxxxxx>
>> Tested-by: Heiko Stuebner <heiko@xxxxxxxxx>
>> Signed-off-by: Dao Lu <daolu@xxxxxxxxxxxx>
>> ---
>>
>> v1 -> v2:
>>  Remove the usage of static branch, use PAUSE if toolchain supports it
>> v2 -> v3:
>>  Added the static branch back, cpu_relax() behavior is kept the same for
>> systems that do not support ZiHintPause
>> v3 -> v4:
>>  Adopted the newly added unified static keys for extensions
>> ---
>>  arch/riscv/Makefile                     |  4 ++++
>>  arch/riscv/include/asm/hwcap.h          |  5 +++++
>>  arch/riscv/include/asm/vdso/processor.h | 21 ++++++++++++++++++---
>>  arch/riscv/kernel/cpu.c                 |  1 +
>>  arch/riscv/kernel/cpufeature.c          |  1 +
>>  5 files changed, 29 insertions(+), 3 deletions(-)
>>
>> diff --git a/arch/riscv/Makefile b/arch/riscv/Makefile
>> index 34cf8a598617..6ddacc6f44b9 100644
>> --- a/arch/riscv/Makefile
>> +++ b/arch/riscv/Makefile
>> @@ -56,6 +56,10 @@ riscv-march-$(CONFIG_RISCV_ISA_C)  := $(riscv-march-y)c
>>  toolchain-need-zicsr-zifencei := $(call cc-option-yn, -march=$(riscv-march-y)_zicsr_zifencei)
>>  riscv-march-$(toolchain-need-zicsr-zifencei) := $(riscv-march-y)_zicsr_zifencei
>>
>> +# Check if the toolchain supports Zihintpause extension
>> +toolchain-supports-zihintpause := $(call cc-option-yn, -march=$(riscv-march-y)_zihintpause)
>> +riscv-march-$(toolchain-supports-zihintpause) := $(riscv-march-y)_zihintpause
>> +
>>  KBUILD_CFLAGS += -march=$(subst fd,,$(riscv-march-y))
>>  KBUILD_AFLAGS += -march=$(riscv-march-y)
>>
>> diff --git a/arch/riscv/include/asm/hwcap.h b/arch/riscv/include/asm/hwcap.h
>> index e48eebdd2631..dc47019a0b38 100644
>> --- a/arch/riscv/include/asm/hwcap.h
>> +++ b/arch/riscv/include/asm/hwcap.h
>> @@ -8,6 +8,7 @@
>>  #ifndef _ASM_RISCV_HWCAP_H
>>  #define _ASM_RISCV_HWCAP_H
>>
>> +#include <asm/errno.h>
>>  #include <linux/bits.h>
>>  #include <uapi/asm/hwcap.h>
>>
>> @@ -54,6 +55,7 @@ extern unsigned long elf_hwcap;
>>  enum riscv_isa_ext_id {
>>       RISCV_ISA_EXT_SSCOFPMF = RISCV_ISA_EXT_BASE,
>>       RISCV_ISA_EXT_SVPBMT,
>> +     RISCV_ISA_EXT_ZIHINTPAUSE,
>>       RISCV_ISA_EXT_ID_MAX = RISCV_ISA_EXT_MAX,
>>  };
>>
>> @@ -64,6 +66,7 @@ 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,
>>  };
>>
>> @@ -83,6 +86,8 @@ 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;
>>       }
>> diff --git a/arch/riscv/include/asm/vdso/processor.h b/arch/riscv/include/asm/vdso/processor.h
>> index 134388cbaaa1..1e4f8b4aef79 100644
>> --- a/arch/riscv/include/asm/vdso/processor.h
>> +++ b/arch/riscv/include/asm/vdso/processor.h
>> @@ -4,15 +4,30 @@
>>
>>  #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.
>> +              */
>> +#ifdef __riscv_zihintpause
>> +             __asm__ __volatile__ ("pause");
>> +#else
>> +             /* Encoding of the pause instruction */
>> +             __asm__ __volatile__ (".4byte 0x100000F");
>> +#endif
>> +     }
>>       barrier();
>>  }
>>
>> diff --git a/arch/riscv/kernel/cpu.c b/arch/riscv/kernel/cpu.c
>> index fba9e9f46a8c..a123e92b14dd 100644
>> --- a/arch/riscv/kernel/cpu.c
>> +++ b/arch/riscv/kernel/cpu.c
>> @@ -89,6 +89,7 @@ int riscv_of_parent_hartid(struct device_node *node)
>>  static struct riscv_isa_ext_data isa_ext_arr[] = {
>>       __RISCV_ISA_EXT_DATA(sscofpmf, RISCV_ISA_EXT_SSCOFPMF),
>>       __RISCV_ISA_EXT_DATA(svpbmt, RISCV_ISA_EXT_SVPBMT),
>> +     __RISCV_ISA_EXT_DATA(zihintpause, RISCV_ISA_EXT_ZIHINTPAUSE),
>>       __RISCV_ISA_EXT_DATA("", RISCV_ISA_EXT_MAX),
>>  };
>>
>> diff --git a/arch/riscv/kernel/cpufeature.c b/arch/riscv/kernel/cpufeature.c
>> index 1b3ec44e25f5..708df2c0bc34 100644
>> --- a/arch/riscv/kernel/cpufeature.c
>> +++ b/arch/riscv/kernel/cpufeature.c
>> @@ -198,6 +198,7 @@ void __init riscv_fill_hwcap(void)
>>                       } else {
>>                               SET_ISA_EXT_MAP("sscofpmf", RISCV_ISA_EXT_SSCOFPMF);
>>                               SET_ISA_EXT_MAP("svpbmt", RISCV_ISA_EXT_SVPBMT);
>> +                             SET_ISA_EXT_MAP("zihintpause", RISCV_ISA_EXT_ZIHINTPAUSE);
>>                       }
>>  #undef SET_ISA_EXT_MAP
>>               }
> 
> Thanks, this is on for-next.  It needs a sparse patch, which I put in as a link.

This breaks the C=1 build for all toolchains, not just new ones as your sparse
patch suggests. I amn't 100% what my CI is running, but I replicated this on
my own machine with:

sparse --version
0.6.4 (Ubuntu: 0.6.4-2)

---8<---
   YACC    scripts/dtc/dtc-parser.tab.[ch]
   HOSTCC  scripts/dtc/libfdt/fdt.o
   HOSTCC  scripts/dtc/libfdt/fdt_ro.o
   HOSTCC  scripts/dtc/libfdt/fdt_wip.o
   UPD     include/generated/uapi/linux/version.h
   HOSTCC  scripts/dtc/libfdt/fdt_sw.o
   UPD     include/config/kernel.release
   HOSTCC  scripts/dtc/libfdt/fdt_rw.o
   HOSTCC  scripts/dtc/libfdt/fdt_strerror.o
   HOSTCC  scripts/dtc/libfdt/fdt_empty_tree.o
   HOSTCC  scripts/dtc/libfdt/fdt_addresses.o
   HOSTCC  scripts/dtc/libfdt/fdt_overlay.o
   HOSTCC  scripts/dtc/fdtoverlay.o
   HOSTCC  scripts/dtc/dtc-lexer.lex.o
   HOSTCC  scripts/dtc/dtc-parser.tab.o
   UPD     include/generated/utsrelease.h
   HOSTLD  scripts/dtc/fdtoverlay
   HOSTLD  scripts/dtc/dtc
   HOSTCC  scripts/kallsyms
   HOSTCC  scripts/sorttable
   HOSTCC  scripts/asn1_compiler
   HOSTCC  scripts/selinux/genheaders/genheaders
   HOSTCC  scripts/selinux/mdp/mdp
   CC      scripts/mod/empty.o
   HOSTCC  scripts/mod/mk_elfconfig
   CC      scripts/mod/devicetable-offsets.s
   CHECK   ../scripts/mod/empty.c
invalid argument to '-march': '_zihintpause'

make[2]: *** [../scripts/Makefile.build:250: scripts/mod/empty.o] Error 1
make[2]: *** Deleting file 'scripts/mod/empty.o'
make[2]: *** Waiting for unfinished jobs....
make[1]: *** [/mnt/automation/corp/workspace/ux-test_upstream-next-develop-cd@2/linux/Makefile:1287: prepare0] Error 2
make[1]: Leaving directory '/mnt/automation/corp/workspace/ux-test_upstream-next-develop-cd@2/linux/builddir'
make: *** [Makefile:231: __sub-make] Error 2