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

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Message-ID: <CAJF2gTTBweJTVw1m1WuJ6LNKrB_U83sdtkiZAk-NLND6sqdMZQ@mail.gmail.com>
Date: Fri, 22 Dec 2023 12:43:22 +0800
From: Guo Ren <guoren@...nel.org>
To: Charlie Jenkins <charlie@...osinc.com>
Cc: Palmer Dabbelt <palmer@...belt.com>, Conor Dooley <conor@...nel.org>, 
	Samuel Holland <samuel.holland@...ive.com>, David Laight <David.Laight@...lab.com>, 
	Xiao Wang <xiao.w.wang@...el.com>, Evan Green <evan@...osinc.com>, 
	linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org, 
	linux-arch@...r.kernel.org, Paul Walmsley <paul.walmsley@...ive.com>, 
	Albert Ou <aou@...s.berkeley.edu>, Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH v13 2/5] riscv: Add static key for misaligned accesses

On Fri, Dec 22, 2023 at 9:37 AM Charlie Jenkins <charlie@...osinc.com> wrote:
>
> 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@...osinc.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@...osinc.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 mean in:
arch/riscv/kernel/cpufeature.c: riscv_online_cpu()

Yes,"adjust the static branch when offline cpus come online ..."

>
> 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.
Em...  stop_machine may be not necessary.

>
> - 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



-- 
Best Regards
 Guo Ren