[External] Re: [PATCH v3 1/2] riscv: process: Introduce idle thread using Zawrs extension

Wed Sep 25 08:01:58 PDT 2024

Hi Andrew,

Thanks a lot for your reply.

On Wed, Sep 25, 2024 at 9:54 PM Andrew Jones <ajones at ventanamicro.com> wrote:
>
> On Wed, Sep 25, 2024 at 09:15:46PM GMT, Xu Lu wrote:
> > The Zawrs extension introduces a new instruction WRS.NTO, which will
> > register a reservation set and causes the hart to temporarily stall
> > execution in a low-power state until a store occurs to the reservation
> > set or an interrupt is observed.
> >
> > This commit implements new version of idle thread for RISC-V via Zawrs
> > extension.
> >
> > Signed-off-by: Xu Lu <luxu.kernel at bytedance.com>
> > Reviewed-by: Hangjing Li <lihangjing at bytedance.com>
> > Reviewed-by: Liang Deng <dengliang.1214 at bytedance.com>
> > Reviewed-by: Wen Chai <chaiwen.cc at bytedance.com>
> > ---
> >  arch/riscv/Kconfig                 | 10 ++++++++
> >  arch/riscv/include/asm/cpuidle.h   | 11 +-------
> >  arch/riscv/include/asm/processor.h | 18 +++++++++++++
> >  arch/riscv/kernel/cpu.c            |  5 ++++
> >  arch/riscv/kernel/process.c        | 41 +++++++++++++++++++++++++++++-
> >  5 files changed, 74 insertions(+), 11 deletions(-)
> >
> > diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> > index 939ea7f6a228..56cf6000d286 100644
> > --- a/arch/riscv/Kconfig
> > +++ b/arch/riscv/Kconfig
> > @@ -23,6 +23,7 @@ config RISCV
> >       select ARCH_ENABLE_SPLIT_PMD_PTLOCK if PGTABLE_LEVELS > 2
> >       select ARCH_ENABLE_THP_MIGRATION if TRANSPARENT_HUGEPAGE
> >       select ARCH_HAS_BINFMT_FLAT
> > +     select ARCH_HAS_CPU_FINALIZE_INIT
> >       select ARCH_HAS_CURRENT_STACK_POINTER
> >       select ARCH_HAS_DEBUG_VIRTUAL if MMU
> >       select ARCH_HAS_DEBUG_VM_PGTABLE
> > @@ -1153,6 +1154,15 @@ endmenu # "Power management options"
> >
> >  menu "CPU Power Management"
> >
> > +config RISCV_ZAWRS_IDLE
> > +     bool "Idle thread using ZAWRS extensions"
> > +     depends on RISCV_ISA_ZAWRS
> > +     default y
> > +     help
> > +             Adds support to implement idle thread using ZAWRS extension.
> > +
> > +             If you don't know what to do here, say Y.
> > +
> >  source "drivers/cpuidle/Kconfig"
> >
> >  source "drivers/cpufreq/Kconfig"
> > diff --git a/arch/riscv/include/asm/cpuidle.h b/arch/riscv/include/asm/cpuidle.h
> > index 71fdc607d4bc..94c9ecb46571 100644
> > --- a/arch/riscv/include/asm/cpuidle.h
> > +++ b/arch/riscv/include/asm/cpuidle.h
> > @@ -10,15 +10,6 @@
> >  #include <asm/barrier.h>
> >  #include <asm/processor.h>
> >
> > -static inline void cpu_do_idle(void)
> > -{
> > -     /*
> > -      * Add mb() here to ensure that all
> > -      * IO/MEM accesses are completed prior
> > -      * to entering WFI.
> > -      */
> > -     mb();
> > -     wait_for_interrupt();
> > -}
> > +void cpu_do_idle(void);
> >
> >  #endif
> > diff --git a/arch/riscv/include/asm/processor.h b/arch/riscv/include/asm/processor.h
> > index efa1b3519b23..d0dcdb7e7392 100644
> > --- a/arch/riscv/include/asm/processor.h
> > +++ b/arch/riscv/include/asm/processor.h
> > @@ -12,6 +12,7 @@
> >
> >  #include <vdso/processor.h>
> >
> > +#include <asm/insn-def.h>
> >  #include <asm/ptrace.h>
> >
> >  #define arch_get_mmap_end(addr, len, flags)                  \
> > @@ -148,6 +149,21 @@ static inline void wait_for_interrupt(void)
> >       __asm__ __volatile__ ("wfi");
> >  }
> >
> > +static inline void wrs_nto(unsigned long *addr)
> > +{
> > +     int val;
> > +
> > +     __asm__ __volatile__(
> > +#ifdef CONFIG_64BIT
> > +                     "lr.d %[p], %[v]\n\t"
> > +#else
> > +                     "lr.w %[p], %[v]\n\t"
> > +#endif
>
> val is always 32-bit since it's an int. We should always use lr.w.

The 'int val' is a mistake here. The val can be an unsigned long
(thread_info->flags) when CONFIG_SMP is disabled. So the lr.d is
necessary. I will update the declaration of val from 'int' to
'unsigned long'.

>
> > +                     ZAWRS_WRS_NTO "\n\t"
> > +                     : [p] "=&r" (val), [v] "+A" (*addr)
>
> What do 'p' and 'v' represent? If they are pointer and value then they're
> backwards. I would just spell them out [val] and [addr].

Great insight. I will refine the name here to make it more readable.

>
> > +                     : : "memory");
> > +}
> > +
> >  extern phys_addr_t dma32_phys_limit;
> >
> >  struct device_node;
> > @@ -177,6 +193,8 @@ extern int set_unalign_ctl(struct task_struct *tsk, unsigned int val);
> >  #define RISCV_SET_ICACHE_FLUSH_CTX(arg1, arg2)       riscv_set_icache_flush_ctx(arg1, arg2)
> >  extern int riscv_set_icache_flush_ctx(unsigned long ctx, unsigned long per_thread);
> >
> > +extern void select_idle_routine(void);
> > +
> >  #endif /* __ASSEMBLY__ */
> >
> >  #endif /* _ASM_RISCV_PROCESSOR_H */
> > diff --git a/arch/riscv/kernel/cpu.c b/arch/riscv/kernel/cpu.c
> > index f6b13e9f5e6c..97a7144fa6cd 100644
> > --- a/arch/riscv/kernel/cpu.c
> > +++ b/arch/riscv/kernel/cpu.c
> > @@ -23,6 +23,11 @@ bool arch_match_cpu_phys_id(int cpu, u64 phys_id)
> >       return phys_id == cpuid_to_hartid_map(cpu);
> >  }
> >
> > +void __init arch_cpu_finalize_init(void)
> > +{
> > +     select_idle_routine();
> > +}
>
> Is there a reason we need to do this at arch_cpu_finalize_init() time?
> This seems like the type of thing we have typically done at the bottom of
> setup_arch().

Actually, there is no special reason here. Just imitated the placement
of x86. It works well too if we put it at the end of setup_arch().

>
> > +
> >  /*
> >   * Returns the hart ID of the given device tree node, or -ENODEV if the node
> >   * isn't an enabled and valid RISC-V hart node.
> > diff --git a/arch/riscv/kernel/process.c b/arch/riscv/kernel/process.c
> > index e4bc61c4e58a..77769965609e 100644
> > --- a/arch/riscv/kernel/process.c
> > +++ b/arch/riscv/kernel/process.c
> > @@ -15,6 +15,7 @@
> >  #include <linux/tick.h>
> >  #include <linux/ptrace.h>
> >  #include <linux/uaccess.h>
> > +#include <linux/static_call.h>
> >
> >  #include <asm/unistd.h>
> >  #include <asm/processor.h>
> > @@ -35,11 +36,49 @@ EXPORT_SYMBOL(__stack_chk_guard);
> >
> >  extern asmlinkage void ret_from_fork(void);
> >
> > -void noinstr arch_cpu_idle(void)
> > +static __cpuidle void default_idle(void)
> > +{
> > +     /*
> > +      * Add mb() here to ensure that all
> > +      * IO/MEM accesses are completed prior
> > +      * to entering WFI.
> > +      */
> > +     mb();
> > +     wait_for_interrupt();
> > +}
> > +
> > +static __cpuidle void wrs_idle(void)
> > +{
> > +     /*
> > +      * Add mb() here to ensure that all
> > +      * IO/MEM accesses are completed prior
> > +      * to entering WRS.NTO.
> > +      */
> > +     mb();
> > +     wrs_nto(&current_thread_info()->flags);
> > +}
> > +
> > +DEFINE_STATIC_CALL_NULL(riscv_idle, default_idle);
> > +
> > +void __cpuidle cpu_do_idle(void)
> > +{
> > +     static_call(riscv_idle)();
> > +}
> > +
> > +void __cpuidle arch_cpu_idle(void)
>
> Switching the section of this from '.noinstr.text' to 'cpuidle.text'
> should probably be a separate patch.
>
> >  {
> >       cpu_do_idle();
> >  }
> >
> > +void __init select_idle_routine(void)
> > +{
> > +     if (IS_ENABLED(CONFIG_RISCV_ZAWRS_IDLE) &&
> > +                     riscv_has_extension_likely(RISCV_ISA_EXT_ZAWRS))
> > +             static_call_update(riscv_idle, wrs_idle);
> > +     else
> > +             static_call_update(riscv_idle, default_idle);
>
> Do we need this 'else'? Can't we set the default at DEFINE_STATIC_CALL*
> time?

Yes, the 'else' branch can be canceled if we set the default idle
function to 'wfi' one using DEFINE_STATIC_CALL. Just not sure which
code looks better.

>
> > +}
> > +
> >  int set_unalign_ctl(struct task_struct *tsk, unsigned int val)
> >  {
> >       if (!unaligned_ctl_available())
> > --
> > 2.20.1
> >
>
> Thanks,
> drew

Best regards,

Xu Lu.