linux-kernel - Re: [PATCH v3 0/8] riscv: Add Zalasr ISA extension support

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Message-ID: <aM0qlTNPiaQRY2Nv@andrea>
Date: Fri, 19 Sep 2025 12:04:05 +0200
From: Andrea Parri <parri.andrea@...il.com>
To: Xu Lu <luxu.kernel@...edance.com>
Cc: corbet@....net, robh@...nel.org, krzk+dt@...nel.org,
	conor+dt@...nel.org, paul.walmsley@...ive.com, palmer@...belt.com,
	aou@...s.berkeley.edu, alex@...ti.fr, will@...nel.org,
	peterz@...radead.org, boqun.feng@...il.com, mark.rutland@....com,
	ajones@...tanamicro.com, brs@...osinc.com, anup@...infault.org,
	atish.patra@...ux.dev, pbonzini@...hat.com, shuah@...nel.org,
	devicetree@...r.kernel.org, linux-riscv@...ts.infradead.org,
	linux-kernel@...r.kernel.org, apw@...onical.com, joe@...ches.com,
	linux-doc@...r.kernel.org, kvm@...r.kernel.org,
	kvm-riscv@...ts.infradead.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v3 0/8] riscv: Add Zalasr ISA extension support

On Fri, Sep 19, 2025 at 03:37:06PM +0800, Xu Lu wrote:
> This patch adds support for the Zalasr ISA extension, which supplies the
> real load acquire/store release instructions.
> 
> The specification can be found here:
> https://github.com/riscv/riscv-zalasr/blob/main/chapter2.adoc
> 
> This patch seires has been tested with ltp on Qemu with Brensan's zalasr
> support patch[1].
> 
> Some false positive spacing error happens during patch checking. Thus I
> CCed maintainers of checkpatch.pl as well.
> 
> [1] https://lore.kernel.org/all/CAGPSXwJEdtqW=nx71oufZp64nK6tK=0rytVEcz4F-gfvCOXk2w@mail.gmail.com/
> 
> v3:
>  - Apply acquire/release semantics to arch_xchg/arch_cmpxchg operations
>  so as to ensure FENCE.TSO ordering between operations which precede the
>  UNLOCK+LOCK sequence and operations which follow the sequence. Thanks
>  to Andrea.
>  - Support hwprobe of Zalasr.
>  - Allow Zalasr extensions for Guest/VM.
> 
> v2:
>  - Adjust the order of Zalasr and Zalrsc in dt-bindings. Thanks to
>  Conor.
> 
> Xu Lu (8):
>   riscv: add ISA extension parsing for Zalasr
>   dt-bindings: riscv: Add Zalasr ISA extension description
>   riscv: hwprobe: Export Zalasr extension
>   riscv: Introduce Zalasr instructions
>   riscv: Use Zalasr for smp_load_acquire/smp_store_release
>   riscv: Apply acquire/release semantics to arch_xchg/arch_cmpxchg
>     operations
>   RISC-V: KVM: Allow Zalasr extensions for Guest/VM
>   KVM: riscv: selftests: Add Zalasr extensions to get-reg-list test
> 
>  Documentation/arch/riscv/hwprobe.rst          |   5 +-
>  .../devicetree/bindings/riscv/extensions.yaml |   5 +
>  arch/riscv/include/asm/atomic.h               |   6 -
>  arch/riscv/include/asm/barrier.h              |  91 ++++++++++--
>  arch/riscv/include/asm/cmpxchg.h              | 136 ++++++++----------
>  arch/riscv/include/asm/hwcap.h                |   1 +
>  arch/riscv/include/asm/insn-def.h             |  79 ++++++++++
>  arch/riscv/include/uapi/asm/hwprobe.h         |   1 +
>  arch/riscv/include/uapi/asm/kvm.h             |   1 +
>  arch/riscv/kernel/cpufeature.c                |   1 +
>  arch/riscv/kernel/sys_hwprobe.c               |   1 +
>  arch/riscv/kvm/vcpu_onereg.c                  |   2 +
>  .../selftests/kvm/riscv/get-reg-list.c        |   4 +
>  13 files changed, 242 insertions(+), 91 deletions(-)

I wouldn't have rushed this submission while the discussion on v2 seems
so much alive;  IAC, to add and link to that discussion, this version
(not a review, just looking at this diff stat) is changing the fastpath

  read_unlock()
  read_lock()

from something like

  fence rw,w
  amodadd.w
  amoadd.w
  fence r,rw

to

  fence rw,rw
  amoadd.w
  amoadd.w
  fence rw,rw

no matter Zalasr or !Zalasr.  Similarly for other atomic operations with
release or acquire semantics.  I guess the change was not intentional?
If it was intentional, it should be properly mentioned in the changelog.

  Andrea