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

Andrew Jones
2022-08-16T16:19:07Z

On Tue, Aug 16, 2022 at 04:04:06PM +0000, Conor.Dooley@microchip.com wrote:
> 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/

Hi Conor,

I have a patch for this which I'll send in just a second.

Thanks,
drew

>
> 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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