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From: Guo Ren <guoren@kernel.org>
Date: Thu, 30 May 2024 09:54:51 +0800
Message-ID: <CAJF2gTRWSZsD3vDcXvawCxt665PZcbwurUqXx3juaoZaDrdttQ@mail.gmail.com>
Subject: Re: [PATCH 7/7] riscv: Add qspinlock support based on Zabha extension
To: Alexandre Ghiti <alexghiti@rivosinc.com>
Cc: Jonathan Corbet <corbet@lwn.net>, Paul Walmsley <paul.walmsley@sifive.com>, 
	Palmer Dabbelt <palmer@dabbelt.com>, Albert Ou <aou@eecs.berkeley.edu>, 
	Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@redhat.com>, Will Deacon <will@kernel.org>, 
	Waiman Long <longman@redhat.com>, Boqun Feng <boqun.feng@gmail.com>, Arnd Bergmann <arnd@arndb.de>, 
	Leonardo Bras <leobras@redhat.com>, linux-doc@vger.kernel.org, 
	linux-kernel@vger.kernel.org, linux-riscv@lists.infradead.org, 
	linux-arch@vger.kernel.org
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On Wed, May 29, 2024 at 9:03=E2=80=AFPM Alexandre Ghiti <alexghiti@rivosinc=
com> wrote:
>
> Hi Guo,
>
> On Wed, May 29, 2024 at 11:24=E2=80=AFAM Guo Ren <guoren@kernel.org> wrot=
e:
> >
> > On Tue, May 28, 2024 at 11:18=E2=80=AFPM Alexandre Ghiti <alexghiti@riv=
osinc.com> wrote:
> > >
> > > In order to produce a generic kernel, a user can select
> > > CONFIG_QUEUED_SPINLOCKS which will fallback at runtime to the ticket
> > > spinlock implementation if Zabha is not present.
> > >
> > > Note that we can't use alternatives here because the discovery of
> > > extensions is done too late and we need to start with the qspinlock
> > > implementation because the ticket spinlock implementation would pollu=
te
> > > the spinlock value, so let's use static keys.
> > >
> > > This is largely based on Guo's work and Leonardo reviews at [1].
> > >
> > > Link: https://lore.kernel.org/linux-riscv/20231225125847.2778638-1-gu=
oren@kernel.org/ [1]
> > > Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
> > > ---
> > >  .../locking/queued-spinlocks/arch-support.txt |  2 +-
> > >  arch/riscv/Kconfig                            |  1 +
> > >  arch/riscv/include/asm/Kbuild                 |  4 +-
> > >  arch/riscv/include/asm/spinlock.h             | 39 +++++++++++++++++=
++
> > >  arch/riscv/kernel/setup.c                     | 18 +++++++++
> > >  include/asm-generic/qspinlock.h               |  2 +
> > >  include/asm-generic/ticket_spinlock.h         |  2 +
> > >  7 files changed, 66 insertions(+), 2 deletions(-)
> > >  create mode 100644 arch/riscv/include/asm/spinlock.h
> > >
> > > diff --git a/Documentation/features/locking/queued-spinlocks/arch-sup=
port.txt b/Documentation/features/locking/queued-spinlocks/arch-support.txt
> > > index 22f2990392ff..cf26042480e2 100644
> > > --- a/Documentation/features/locking/queued-spinlocks/arch-support.tx=
t
> > > +++ b/Documentation/features/locking/queued-spinlocks/arch-support.tx=
t
> > > @@ -20,7 +20,7 @@
> > >      |    openrisc: |  ok  |
> > >      |      parisc: | TODO |
> > >      |     powerpc: |  ok  |
> > > -    |       riscv: | TODO |
> > > +    |       riscv: |  ok  |
> > >      |        s390: | TODO |
> > >      |          sh: | TODO |
> > >      |       sparc: |  ok  |
> > > diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> > > index 184a9edb04e0..ccf1703edeb9 100644
> > > --- a/arch/riscv/Kconfig
> > > +++ b/arch/riscv/Kconfig
> > > @@ -59,6 +59,7 @@ config RISCV
> > >         select ARCH_SUPPORTS_SHADOW_CALL_STACK if HAVE_SHADOW_CALL_ST=
ACK
> > >         select ARCH_USE_MEMTEST
> > >         select ARCH_USE_QUEUED_RWLOCKS
> > > +       select ARCH_USE_QUEUED_SPINLOCKS if TOOLCHAIN_HAS_ZABHA
> > Using qspinlock or not depends on real hardware capabilities, not the
> > compiler flag. That's why I introduced combo-spinlock, ticket-spinlock
> > & qspinlock three Kconfigs, and the combo-spinlock would compat all
> > hardware platforms but waste some qspinlock code size.
>
> You're right, and I think your comment matches what Conor mentioned
> about the lack of clarity with some extensions: TOOLCHAIN_HAS_ZABHA
> will allow a platform with Zabha capability to use qspinlocks. But if
> the hardware does not, it will fallback to the ticket spinlocks.
>
> But I agree that looking at the config alone may be misleading, even
> though it will work as expected at runtime. So I agree with you:
> unless anyone is strongly against the combo spinlocks, I will do what
> you suggest and add them.
The problem with the v12 combo-spinlock is using a static_branch
instead of the full ALTERNATIVE. Frankly, that's a bad example that
costs more code space. I found that your cmpxchg32/64 also uses a
condition branch, which has a similar problem, right?

Anyway, your patch series inspired me to update the v13
combo-spinlock. My plan is:
1. Separate native-qspinlock out of paravirt-qspinlock.
2. Re-design an ALTERNATIVE(asm) code instead of static_branch generic
ticket-lock or qspinlock.

What do you think?


>
> Thanks again for your initial work,
>
> Alex
>
> >
> > >         select ARCH_USES_CFI_TRAPS if CFI_CLANG
> > >         select ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH if SMP && MMU
> > >         select ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT if MMU
> > > diff --git a/arch/riscv/include/asm/Kbuild b/arch/riscv/include/asm/K=
build
> > > index 504f8b7e72d4..ad72f2bd4cc9 100644
> > > --- a/arch/riscv/include/asm/Kbuild
> > > +++ b/arch/riscv/include/asm/Kbuild
> > > @@ -2,10 +2,12 @@
> > >  generic-y +=3D early_ioremap.h
> > >  generic-y +=3D flat.h
> > >  generic-y +=3D kvm_para.h
> > > +generic-y +=3D mcs_spinlock.h
> > >  generic-y +=3D parport.h
> > > -generic-y +=3D spinlock.h
> > >  generic-y +=3D spinlock_types.h
> > > +generic-y +=3D ticket_spinlock.h
> > >  generic-y +=3D qrwlock.h
> > >  generic-y +=3D qrwlock_types.h
> > > +generic-y +=3D qspinlock.h
> > >  generic-y +=3D user.h
> > >  generic-y +=3D vmlinux.lds.h
> > > diff --git a/arch/riscv/include/asm/spinlock.h b/arch/riscv/include/a=
sm/spinlock.h
> > > new file mode 100644
> > > index 000000000000..e00429ac20ed
> > > --- /dev/null
> > > +++ b/arch/riscv/include/asm/spinlock.h
> > > @@ -0,0 +1,39 @@
> > > +/* SPDX-License-Identifier: GPL-2.0 */
> > > +
> > > +#ifndef __ASM_RISCV_SPINLOCK_H
> > > +#define __ASM_RISCV_SPINLOCK_H
> > > +
> > > +#ifdef CONFIG_QUEUED_SPINLOCKS
> > > +#define _Q_PENDING_LOOPS       (1 << 9)
> > > +
> > > +#define __no_arch_spinlock_redefine
> > > +#include <asm/ticket_spinlock.h>
> > > +#include <asm/qspinlock.h>
> > > +#include <asm/alternative.h>
> > > +
> > > +DECLARE_STATIC_KEY_TRUE(qspinlock_key);
> > > +
> > > +#define SPINLOCK_BASE_DECLARE(op, type, type_lock)                  =
   \
> > > +static __always_inline type arch_spin_##op(type_lock lock)          =
   \
> > > +{                                                                   =
   \
> > > +       if (static_branch_unlikely(&qspinlock_key))                  =
   \
> > > +               return queued_spin_##op(lock);                       =
   \
> > > +       return ticket_spin_##op(lock);                               =
   \
> > > +}
> > > +
> > > +SPINLOCK_BASE_DECLARE(lock, void, arch_spinlock_t *)
> > > +SPINLOCK_BASE_DECLARE(unlock, void, arch_spinlock_t *)
> > > +SPINLOCK_BASE_DECLARE(is_locked, int, arch_spinlock_t *)
> > > +SPINLOCK_BASE_DECLARE(is_contended, int, arch_spinlock_t *)
> > > +SPINLOCK_BASE_DECLARE(trylock, bool, arch_spinlock_t *)
> > > +SPINLOCK_BASE_DECLARE(value_unlocked, int, arch_spinlock_t)
> > > +
> > > +#else
> > > +
> > > +#include <asm/ticket_spinlock.h>
> > > +
> > > +#endif
> > > +
> > > +#include <asm/qrwlock.h>
> > > +
> > > +#endif /* __ASM_RISCV_SPINLOCK_H */
> > > diff --git a/arch/riscv/kernel/setup.c b/arch/riscv/kernel/setup.c
> > > index 4f73c0ae44b2..31ce75522fd4 100644
> > > --- a/arch/riscv/kernel/setup.c
> > > +++ b/arch/riscv/kernel/setup.c
> > > @@ -244,6 +244,23 @@ static void __init parse_dtb(void)
> > >  #endif
> > >  }
> > >
> > > +DEFINE_STATIC_KEY_TRUE(qspinlock_key);
> > > +EXPORT_SYMBOL(qspinlock_key);
> > > +
> > > +static void __init riscv_spinlock_init(void)
> > > +{
> > > +       asm goto(ALTERNATIVE("nop", "j %[qspinlock]", 0, RISCV_ISA_EX=
T_ZABHA, 1)
> > > +                : : : : qspinlock);
> > > +
> > > +       static_branch_disable(&qspinlock_key);
> > > +       pr_info("Ticket spinlock: enabled\n");
> > > +
> > > +       return;
> > > +
> > > +qspinlock:
> > > +       pr_info("Queued spinlock: enabled\n");
> > > +}
> > > +
> > >  extern void __init init_rt_signal_env(void);
> > >
> > >  void __init setup_arch(char **cmdline_p)
> > > @@ -295,6 +312,7 @@ void __init setup_arch(char **cmdline_p)
> > >         riscv_set_dma_cache_alignment();
> > >
> > >         riscv_user_isa_enable();
> > > +       riscv_spinlock_init();
> > >  }
> > >
> > >  bool arch_cpu_is_hotpluggable(int cpu)
> > > diff --git a/include/asm-generic/qspinlock.h b/include/asm-generic/qs=
pinlock.h
> > > index 0655aa5b57b2..bf47cca2c375 100644
> > > --- a/include/asm-generic/qspinlock.h
> > > +++ b/include/asm-generic/qspinlock.h
> > > @@ -136,6 +136,7 @@ static __always_inline bool virt_spin_lock(struct=
 qspinlock *lock)
> > >  }
> > >  #endif
> > >
> > > +#ifndef __no_arch_spinlock_redefine
> > >  /*
> > >   * Remapping spinlock architecture specific functions to the corresp=
onding
> > >   * queued spinlock functions.
> > > @@ -146,5 +147,6 @@ static __always_inline bool virt_spin_lock(struct=
 qspinlock *lock)
> > >  #define arch_spin_lock(l)              queued_spin_lock(l)
> > >  #define arch_spin_trylock(l)           queued_spin_trylock(l)
> > >  #define arch_spin_unlock(l)            queued_spin_unlock(l)
> > > +#endif
> > >
> > >  #endif /* __ASM_GENERIC_QSPINLOCK_H */
> > > diff --git a/include/asm-generic/ticket_spinlock.h b/include/asm-gene=
ric/ticket_spinlock.h
> > > index cfcff22b37b3..325779970d8a 100644
> > > --- a/include/asm-generic/ticket_spinlock.h
> > > +++ b/include/asm-generic/ticket_spinlock.h
> > > @@ -89,6 +89,7 @@ static __always_inline int ticket_spin_is_contended=
(arch_spinlock_t *lock)
> > >         return (s16)((val >> 16) - (val & 0xffff)) > 1;
> > >  }
> > >
> > > +#ifndef __no_arch_spinlock_redefine
> > >  /*
> > >   * Remapping spinlock architecture specific functions to the corresp=
onding
> > >   * ticket spinlock functions.
> > > @@ -99,5 +100,6 @@ static __always_inline int ticket_spin_is_contende=
d(arch_spinlock_t *lock)
> > >  #define arch_spin_lock(l)              ticket_spin_lock(l)
> > >  #define arch_spin_trylock(l)           ticket_spin_trylock(l)
> > >  #define arch_spin_unlock(l)            ticket_spin_unlock(l)
> > > +#endif
> > >
> > >  #endif /* __ASM_GENERIC_TICKET_SPINLOCK_H */
> > > --
> > > 2.39.2
> > >
> >
> >
> > --
> > Best Regards
> >  Guo Ren


--
Best Regards
 Guo Ren