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

[<Conor.Dooley@microchip.com>]

On 16/08/2022 16:54, Palmer Dabbelt wrote:
> 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?

Yes, sorry - I *thought* the binutils in our CI predated Zihintpause but
it doesn't. It (and so would the Zicbom stuff) breaks the builds in our CI
as they run with C=1. I manually patched sparse to get that going again & I
/suspect/ it may have impacted LKP too.

"New" is relative though - it breaks C=1 for anyone running a toolchain
from riscv-collab/riscv-gnu-toolchain.

I guess it's just that Zicbom is newer so not many people will be on a
toolchain that supports that. My GCC doesn't only my clang-15.

> 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.

It is breaking module loading on RISC-V in general from what it seems.
See:
https://lore.kernel.org/linux-riscv/728ecbd5-975a-168e-efab-3c0030be21d5@w6rz.net/

Thanks,
Conor.

> 
>> 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
>>
> 
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