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Date: Wed, 29 May 2024 15:03:00 +0200
From: Alexandre Ghiti <alexghiti@...osinc.com>
To: Guo Ren <guoren@...nel.org>
Cc: Jonathan Corbet <corbet@....net>, Paul Walmsley <paul.walmsley@...ive.com>,
Palmer Dabbelt <palmer@...belt.com>, Albert Ou <aou@...s.berkeley.edu>,
Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
Waiman Long <longman@...hat.com>, Boqun Feng <boqun.feng@...il.com>, Arnd Bergmann <arnd@...db.de>,
Leonardo Bras <leobras@...hat.com>, linux-doc@...r.kernel.org,
linux-kernel@...r.kernel.org, linux-riscv@...ts.infradead.org,
linux-arch@...r.kernel.org
Subject: Re: [PATCH 7/7] riscv: Add qspinlock support based on Zabha extension
Hi Guo,
On Wed, May 29, 2024 at 11:24 AM Guo Ren <guoren@...nel.org> wrote:
>
> On Tue, May 28, 2024 at 11:18 PM Alexandre Ghiti <alexghiti@...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 pollute
> > 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-guoren@kernel.org/ [1]
> > Signed-off-by: Alexandre Ghiti <alexghiti@...osinc.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-support.txt b/Documentation/features/locking/queued-spinlocks/arch-support.txt
> > index 22f2990392ff..cf26042480e2 100644
> > --- a/Documentation/features/locking/queued-spinlocks/arch-support.txt
> > +++ b/Documentation/features/locking/queued-spinlocks/arch-support.txt
> > @@ -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_STACK
> > 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.
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/Kbuild
> > index 504f8b7e72d4..ad72f2bd4cc9 100644
> > --- a/arch/riscv/include/asm/Kbuild
> > +++ b/arch/riscv/include/asm/Kbuild
> > @@ -2,10 +2,12 @@
> > generic-y += early_ioremap.h
> > generic-y += flat.h
> > generic-y += kvm_para.h
> > +generic-y += mcs_spinlock.h
> > generic-y += parport.h
> > -generic-y += spinlock.h
> > generic-y += spinlock_types.h
> > +generic-y += ticket_spinlock.h
> > generic-y += qrwlock.h
> > generic-y += qrwlock_types.h
> > +generic-y += qspinlock.h
> > generic-y += user.h
> > generic-y += vmlinux.lds.h
> > diff --git a/arch/riscv/include/asm/spinlock.h b/arch/riscv/include/asm/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_EXT_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/qspinlock.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 corresponding
> > * 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-generic/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 corresponding
> > * ticket spinlock functions.
> > @@ -99,5 +100,6 @@ static __always_inline int ticket_spin_is_contended(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
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