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From: Conor Dooley <conor.dooley@microchip.com>
To: Guo Ren <guoren@kernel.org>
Cc: <paul.walmsley@sifive.com>, <anup@brainfault.org>,
	<peterz@infradead.org>, <mingo@redhat.com>, <will@kernel.org>,
	<palmer@rivosinc.com>, <longman@redhat.com>,
	<boqun.feng@gmail.com>, <tglx@linutronix.de>,
	<paulmck@kernel.org>, <rostedt@goodmis.org>,
	<rdunlap@infradead.org>, <catalin.marinas@arm.com>,
	<xiaoguang.xing@sophgo.com>, <bjorn@rivosinc.com>,
	<alexghiti@rivosinc.com>, <keescook@chromium.org>,
	<greentime.hu@sifive.com>, <ajones@ventanamicro.com>,
	<jszhang@kernel.org>, <wefu@redhat.com>, <wuwei2016@iscas.ac.cn>,
	<linux-arch@vger.kernel.org>, <linux-riscv@lists.infradead.org>,
	<linux-doc@vger.kernel.org>, <kvm@vger.kernel.org>,
	<virtualization@lists.linux-foundation.org>,
	<linux-csky@vger.kernel.org>, Guo Ren <guoren@linux.alibaba.com>
Subject: Re: [PATCH V10 07/19] riscv: qspinlock: errata: Introduce ERRATA_THEAD_QSPINLOCK
Date: Fri, 4 Aug 2023 11:06:42 +0100	[thread overview]
Message-ID: <20230804-throwaway-requisite-c73ebe3fee8c@wendy> (raw)
In-Reply-To: <CAJF2gTTfLmCe7eDhfPU1qFTBoVZN8oFACEd4NmTyZaAVtdMK-w@mail.gmail.com>


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On Fri, Aug 04, 2023 at 05:53:35PM +0800, Guo Ren wrote:
> On Fri, Aug 4, 2023 at 5:06 PM Conor Dooley <conor.dooley@microchip.com> wrote:
> > On Wed, Aug 02, 2023 at 12:46:49PM -0400, guoren@kernel.org wrote:
> > > From: Guo Ren <guoren@linux.alibaba.com>

> > > diff --git a/arch/riscv/kernel/cpufeature.c b/arch/riscv/kernel/cpufeature.c
> > > index f8dbbe1bbd34..d9694fe40a9a 100644
> > > --- a/arch/riscv/kernel/cpufeature.c
> > > +++ b/arch/riscv/kernel/cpufeature.c
> > > @@ -342,7 +342,8 @@ void __init riscv_fill_hwcap(void)
> > >                * spinlock value, the only way is to change from queued_spinlock to
> > >                * ticket_spinlock, but can not be vice.
> > >                */
> > > -             if (!force_qspinlock) {
> > > +             if (!force_qspinlock &&
> > > +                 !riscv_has_errata_thead_qspinlock()) {
> > >                       set_bit(RISCV_ISA_EXT_XTICKETLOCK, isainfo->isa);
> >
> > Is this a generic vendor extension (lol @ that misnomer) or is it an
> > erratum? Make your mind up please. As has been said on other series, NAK
> > to using march/vendor/imp IDs for feature probing.
>
> The RISCV_ISA_EXT_XTICKETLOCK is a feature extension number,

No, that is not what "ISA_EXT" means, nor what the X in "XTICKETLOCK"
would imply.

The comment above these reads:
  These macros represent the logical IDs of each multi-letter RISC-V ISA
  extension and are used in the ISA bitmap.

> and it's
> set by default for forward-compatible. We also define a vendor
> extension (riscv_has_errata_thead_qspinlock) to force all our
> processors to use qspinlock; others still stay on ticket_lock.

No, "riscv_has_errata_thead_qspinlock()" would be an _erratum_, not a
vendor extension. We need to have a discussion about how to support
non-standard extensions etc, not abuse errata. That discussion has been
started on the v0.7.1 vector patches, but has not made progress yet.

> The only possible changing direction is from qspinlock to ticket_lock
> because ticket_lock would dirty the lock value, which prevents
> changing to qspinlock next. So startup with qspinlock and change to
> ticket_lock before smp up. You also could use cmdline to try qspinlock
> (force_qspinlock).

I don't see what the relevance of this is, sorry. I am only commenting
on how you are deciding that the hardware is capable of using qspinlocks,
I don't intend getting into the detail unless the powers that be deem
this series worthwhile, as I mentioned:
> > I've got some thoughts on other parts of this series too, but I'm not
> > going to spend time on it unless the locking people and Palmer ascent
> > to this series.


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  reply	other threads:[~2023-08-04 10:07 UTC|newest]

Thread overview: 36+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2023-08-02 16:46 [PATCH V10 00/19] riscv: Add Native/Paravirt/CNA qspinlock support guoren
2023-08-02 16:46 ` [PATCH V10 01/19] asm-generic: ticket-lock: Reuse arch_spinlock_t of qspinlock guoren
2023-08-02 16:46 ` [PATCH V10 02/19] asm-generic: ticket-lock: Move into ticket_spinlock.h guoren
2023-08-02 16:46 ` [PATCH V10 03/19] riscv: qspinlock: errata: Add ERRATA_THEAD_WRITE_ONCE fixup guoren
2023-08-02 16:46 ` [PATCH V10 04/19] riscv: qspinlock: Add basic queued_spinlock support guoren
2023-08-11 19:34   ` Waiman Long
2023-08-12  0:18     ` Guo Ren
2023-08-02 16:46 ` [PATCH V10 05/19] riscv: qspinlock: Introduce combo spinlock guoren
2023-08-11 19:51   ` Waiman Long
2023-08-12  0:22     ` Guo Ren
2023-08-02 16:46 ` [PATCH V10 06/19] riscv: qspinlock: Allow force qspinlock from the command line guoren
2023-08-02 16:46 ` [PATCH V10 07/19] riscv: qspinlock: errata: Introduce ERRATA_THEAD_QSPINLOCK guoren
2023-08-04  9:05   ` Conor Dooley
2023-08-04  9:53     ` Guo Ren
2023-08-04 10:06       ` Conor Dooley [this message]
2023-08-05  1:28         ` Guo Ren
2023-08-07  5:23   ` Stefan O'Rear
2023-08-08  2:12     ` Guo Ren
2023-09-13 18:54     ` Palmer Dabbelt
2023-09-13 19:32       ` Waiman Long
2023-09-14  3:31       ` Guo Ren
2023-08-02 16:46 ` [PATCH V10 08/19] riscv: qspinlock: Use new static key for controlling call of virt_spin_lock() guoren
2023-08-02 16:46 ` [PATCH V10 09/19] RISC-V: paravirt: pvqspinlock: Add paravirt qspinlock skeleton guoren
2023-08-02 16:46 ` [PATCH V10 10/19] RISC-V: paravirt: pvqspinlock: KVM: " guoren
2023-08-02 16:46 ` [PATCH V10 11/19] RISC-V: paravirt: pvqspinlock: KVM: Implement kvm_sbi_ext_pvlock_kick_cpu() guoren
2023-08-02 16:46 ` [PATCH V10 12/19] RISC-V: paravirt: pvqspinlock: Add nopvspin kernel parameter guoren
2023-08-02 16:46 ` [PATCH V10 13/19] RISC-V: paravirt: pvqspinlock: Remove unnecessary definitions of cmpxchg & xchg guoren
2023-08-02 16:46 ` [PATCH V10 14/19] RISC-V: paravirt: pvqspinlock: Add xchg8 & cmpxchg_small support guoren
2023-08-02 16:46 ` [PATCH V10 15/19] RISC-V: paravirt: pvqspinlock: Add SBI implementation guoren
2023-08-02 16:46 ` [PATCH V10 16/19] RISC-V: paravirt: pvqspinlock: Add kconfig entry guoren
2023-08-02 16:46 ` [PATCH V10 17/19] RISC-V: paravirt: pvqspinlock: Add trace point for pv_kick/wait guoren
2023-08-02 16:47 ` [PATCH V10 18/19] locking/qspinlock: Move pv_ops into x86 directory guoren
2023-08-11 20:42   ` Waiman Long
2023-08-12  0:24     ` Guo Ren
2023-08-12  0:47       ` Waiman Long
2023-08-02 16:47 ` [PATCH V10 19/19] locking/qspinlock: riscv: Add Compact NUMA-aware lock support guoren

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