Date: Mon, 3 Jun 2024 19:11:24 +0800
From: Guo Ren <guoren@...nel.org>
To: Alexandre Ghiti <alex@...ti.fr>
Cc: Alexandre Ghiti <alexghiti@...osinc.com>, 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

On Mon, Jun 3, 2024 at 5:22 PM Alexandre Ghiti <alex@...ti.fr> wrote:
>
> Hi Guo,
>
> On 31/05/2024 08:42, Guo Ren wrote:
> > On Fri, May 31, 2024 at 2:22 PM Alexandre Ghiti <alexghiti@...osinc.com> wrote:
> >> On Fri, May 31, 2024 at 3:57 AM Guo Ren <guoren@...nel.org> wrote:
> >>> On Thu, May 30, 2024 at 1:30 PM Alexandre Ghiti <alexghiti@...osinc.com> wrote:
> >>>> Hi Guo,
> >>>>
> >>>> On Thu, May 30, 2024 at 3:55 AM Guo Ren <guoren@...nel.org> wrote:
> >>>>> On Wed, May 29, 2024 at 9:03 PM Alexandre Ghiti <alexghiti@...osinc.com> wrote:
> >>>>>> 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.
> >>>>> 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's your plan to make use of alternatives here? The alternatives
> >>>> patching depends on the discovery of the extensions, which is done too
> >>>> late, at least after the first use of a spinlock (the printk
> >>>> spinlock). So you'd need to find a way to first use qspinlocks (but
> >>>> without knowing Zabha is available) and then do the correct patching:
> >>> I do that in v12:
> >>> 1. Use qspinlock as init.
> >>> 2. Change to ticket-lock or not.
> >>> (Only qspinlock -> ticket-lock, No reverse direction)
> >>>
> >>> If there is no contention, Qspinlock is okay for all platforms before
> >>> smp bringup & no-irq environment.
> >>>
> >> Yes, by using static keys not alternatives. My question was: how do
> >> you plan to use alternatives here instead of static keys? To me, it's
> >> not that simple, hence my suggestions in my previous answer.
> > Yes, it's not that simple. The current framework doesn't support that
> > and has two problems:
> > 1. We need to re-implement ticket-lock & qspinlock-fast-path with assembly code.
> > 2. Current alternatives patching only for extensions, but qspinlock is
> > not a formal extension. Could we accept
> > __RISCV_ISA_EXT_DATA(xqspinlock, RISCV_ISA_EXT_XQSPINLOCK)?
>
>
> But the problem is that the alternatives needs to patch the code very
> early in the boot process which is not possible since we don't have the
> list of extensions yet (for ACPI systems), so your
> RISCV_ISA_EXT_XQSPINLOCK proposal would not help.
I think the setup_arch()->apply_boot_alternatives() is okay. I can do
that in v13.

>
> Thanks,
>
> Alex
>
>
> >
> >> Thanks,
> >>
> >> Alex
> >>
> >>>> an idea here could be to add an "init" value to the alternatives and
> >>>> let the patching process do the right thing when the extensions are
> >>>> known.
> >>>>
> >>>> Another solution would be the early discovery of the extensions, but I
> >>>> took a look and it's easy with a device tree, but not with ACPI.
> >>>>
> >>>> Let me know what you plan to do and how I can help!
> >>>>
> >>>> Thanks,
> >>>>
> >>>> Alex
> >>>>
> >>>>> 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/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
> >>>>>
> >>>>>
> >>>>> --
> >>>>> Best Regards
> >>>>>   Guo Ren
> >>>
> >>>
> >>> --
> >>> Best Regards
> >>>   Guo Ren
> >
> >



-- 
Best Regards
 Guo Ren

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