Re: [PATCH v13 2/5] riscv: Add static key for misaligned accesses

[Charlie Jenkins <charlie@rivosinc.com>]

On Fri, Dec 22, 2023 at 08:33:18AM +0800, Guo Ren wrote:
> On Thu, Dec 21, 2023 at 7:38 AM Charlie Jenkins <charlie@rivosinc.com> wrote:
> >
> > Support static branches depending on the value of misaligned accesses.
> > This will be used by a later patch in the series. All cpus must be
> > considered "fast" for this static branch to be flipped.
> >
> > Signed-off-by: Charlie Jenkins <charlie@rivosinc.com>
> > ---
> >  arch/riscv/include/asm/cpufeature.h |  2 ++
> >  arch/riscv/kernel/cpufeature.c      | 30 ++++++++++++++++++++++++++++++
> >  2 files changed, 32 insertions(+)
> >
> > diff --git a/arch/riscv/include/asm/cpufeature.h b/arch/riscv/include/asm/cpufeature.h
> > index a418c3112cd6..7b129e5e2f07 100644
> > --- a/arch/riscv/include/asm/cpufeature.h
> > +++ b/arch/riscv/include/asm/cpufeature.h
> > @@ -133,4 +133,6 @@ static __always_inline bool riscv_cpu_has_extension_unlikely(int cpu, const unsi
> >         return __riscv_isa_extension_available(hart_isa[cpu].isa, ext);
> >  }
> >
> > +DECLARE_STATIC_KEY_FALSE(fast_misaligned_access_speed_key);
> > +
> >  #endif
> > diff --git a/arch/riscv/kernel/cpufeature.c b/arch/riscv/kernel/cpufeature.c
> > index b3785ffc1570..095eb6ebdcaa 100644
> > --- a/arch/riscv/kernel/cpufeature.c
> > +++ b/arch/riscv/kernel/cpufeature.c
> > @@ -10,6 +10,7 @@
> >  #include <linux/bitmap.h>
> >  #include <linux/cpuhotplug.h>
> >  #include <linux/ctype.h>
> > +#include <linux/jump_label.h>
> >  #include <linux/log2.h>
> >  #include <linux/memory.h>
> >  #include <linux/module.h>
> > @@ -728,6 +729,35 @@ void riscv_user_isa_enable(void)
> >                 csr_set(CSR_SENVCFG, ENVCFG_CBZE);
> >  }
> >
> > +DEFINE_STATIC_KEY_FALSE(fast_misaligned_access_speed_key);
> > +
> > +static int set_unaligned_access_static_branches(void)
> > +{
> > +       /*
> > +        * This will be called after check_unaligned_access_all_cpus so the
> > +        * result of unaligned access speed for all cpus will be available.
> > +        */
> > +
> > +       int cpu;
> > +       bool fast_misaligned_access_speed = true;
> > +
> > +       for_each_online_cpu(cpu) {
> Each online_cpu? Is there any offline_cpu that is no
> fast_misaligned_access_speed?

I think instead of checking offline cpus, it would make more sense to
adjust the static branch when offline cpus come online. Since
riscv_online_cpu is called when a new CPU comes online, I can update the
static branch inside of that function.

> 
> Move into your riscv_online_cpu for each CPU, and use stop_machine for
> synchronization.
> 

I do not understand what you mean by "Move into your riscv_online_cpu
for each CPU", but I am assuming you are referring to updating the
static branch inside of riscv_online_cpu.

I believe any race condition that could be solved by stop_machine will
become irrelevent by ensuring that the static branch is updated when a
new cpu comes online. 

- Charlie

> > +               int this_perf = per_cpu(misaligned_access_speed, cpu);
> > +
> > +               if (this_perf != RISCV_HWPROBE_MISALIGNED_FAST) {
> > +                       fast_misaligned_access_speed = false;
> > +                       break;
> > +               }
> > +       }
> > +
> > +       if (fast_misaligned_access_speed)
> > +               static_branch_enable(&fast_misaligned_access_speed_key);
> > +
> > +       return 0;
> > +}
> > +
> > +arch_initcall_sync(set_unaligned_access_static_branches);
> > +
> >  #ifdef CONFIG_RISCV_ALTERNATIVE
> >  /*
> >   * Alternative patch sites consider 48 bits when determining when to patch
> >
> > --
> > 2.43.0
> >
> >
> 
> 
> -- 
> Best Regards
>  Guo Ren