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

Palmer Dabbelt
2022-08-16T15:57:28Z

On Fri, 12 Aug 2022 00:21:40 PDT (-0700), Conor.Dooley@microchip.com wrote:
> On 12/08/2022 07:57, Conor Dooley - M52691 wrote:
>> 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@rivosinc.com 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@sntech.de>
>>>> Tested-by: Heiko Stuebner <heiko@sntech.de>
>>>> Signed-off-by: Dao Lu <daolu@rivosinc.com>
>>>> ---
>>>>
>>>> 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:
>
> Argh, I went poking around and my toolchain's binutils etc is newer than I thought.
> Good for people searching on lore I suppose...
> Sorry!

So just to be clear: you're saying this only breaks with new binutils?
I ask because Dao is also seeing some crashes, if it's breaking
arbitrary builds too then it's a stronger hint to revert it.

> Conor.
>
>>
>> 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
>