linux-kernel - Re: [PATCH v4 00/10] riscv: Add Zalasr ISA extension support

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Message-ID: <aPvz2Pb6RuWnw9Ht@andrea>
Date: Fri, 24 Oct 2025 23:47:04 +0200
From: Andrea Parri <parri.andrea@...il.com>
To: Xu Lu <luxu.kernel@...edance.com>
Cc: corbet@....net, paul.walmsley@...ive.com, palmer@...belt.com,
	aou@...s.berkeley.edu, alex@...ti.fr, robh@...nel.org,
	krzk+dt@...nel.org, conor+dt@...nel.org, will@...nel.org,
	peterz@...radead.org, boqun.feng@...il.com, mark.rutland@....com,
	anup@...infault.org, atish.patra@...ux.dev, pbonzini@...hat.com,
	shuah@...nel.org, ajones@...tanamicro.com, brs@...osinc.com,
	guoren@...nel.org, linux-doc@...r.kernel.org,
	linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
	devicetree@...r.kernel.org, kvm@...r.kernel.org,
	kvm-riscv@...ts.infradead.org, linux-kselftest@...r.kernel.org,
	apw@...onical.com, joe@...ches.com, lukas.bulwahn@...il.com
Subject: Re: [PATCH v4 00/10] riscv: Add Zalasr ISA extension support

On Mon, Oct 20, 2025 at 12:20:46PM +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/
> 
> v4:
>  - Apply acquire/release semantics to arch_atomic operations. Thanks
>  to Andrea.

Perhaps I wasn't clear enough, sorry, but I did mean my suggestion
(specifically, the use of .aq/.rl annotations) to be conditional on
zalasr.  Same observation for xchg/cmpxchg below.  IOW, I really
expected this series to introduce _no changes_ to our lowerings when
!zalasr.  If any !zalasr-changes are needed, I'd suggest isolating
/submitting them in dedicated patches.

But other than that, this looks pretty good to me.  Perhaps something
else to consider for zalasr is our lowering of smp_cond_load_acquire()
(can't spot an actual bug now, but recall the principle "zalasr -> use
.aq/.rl annotations ..."): riscv currently uses the "generic", fence-
based implementation from include/asm-generic/barrier.h; compare that
with e.g. the implementation from arch/arm64/include/asm/barrier.h .

  Andrea


> 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 (10):
>   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: Apply Zalasr to smp_load_acquire/smp_store_release
>   riscv: Apply acquire/release semantics to arch_xchg/arch_cmpxchg
>     operations
>   riscv: Apply acquire/release semantics to arch_atomic operations
>   riscv: Remove arch specific __atomic_acquire/release_fence
>   RISC-V: KVM: Allow Zalasr extensions for Guest/VM
>   RISC-V: KVM: 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               |  70 ++++++++-
>  arch/riscv/include/asm/barrier.h              |  91 +++++++++--
>  arch/riscv/include/asm/cmpxchg.h              | 144 +++++++++---------
>  arch/riscv/include/asm/fence.h                |   4 -
>  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 +
>  14 files changed, 314 insertions(+), 95 deletions(-)
> 
> -- 
> 2.20.1
>