From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 8CBA1C982ED for ; Mon, 21 Sep 2026 12:27:57 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender: Content-Transfer-Encoding:Content-Type:List-Subscribe:List-Help:List-Post: List-Archive:List-Unsubscribe:List-Id:Cc:To:In-Reply-To:References:Message-Id :MIME-Version:Subject:Date:From:Reply-To:Content-ID:Content-Description: Resent-Date:Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID: List-Owner; bh=Qh4iNoLAdkkEzP6gd70urm5BUOY4hcA+b6AA0BPS+uo=; b=h0J0CZllcKul/m s2rAgc5WYaeAuWk801uc5ThEg3IhwSA4vHhgKCovSed/XRnMCt/Rh7ySlHvSN7zI3odyV+2uPasx1 2fgmYjZNDKam/nJ9zhKdUbalYB5kxLm4dYIy2TCJvnKn55e9mM/3MhCoUlJ48k1DTb6dD/bdW/RNC FayKBTvP7jP9S+71zAmWAX6+ImGb/0dT+0AlHSguGuey1/EjxP8UgL+q5E8iSohRWXMRH5oFC4Y2B DFNFQ3LBLpmKn12a8pvcdVl69Cqo1XIuH7EQO9/yEVqczecttuBX7aFLc7nUKab3dFGyogP5AxKmq y+3k+n1tsfYze1/AzOJQ==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.99.1 #2 (Red Hat Linux)) id 1x8d7Y-000000023fa-0xqB; Mon, 21 Sep 2026 12:27:44 +0000 Received: from sea.source.kernel.org ([172.234.252.31]) by bombadil.infradead.org with esmtps (Exim 4.99.1 #2 (Red Hat Linux)) id 1x8d7X-000000023ee-10AH; Mon, 21 Sep 2026 12:27:43 +0000 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) by sea.source.kernel.org (Postfix) with ESMTP id 03BEE43D7D; Mon, 21 Sep 2026 12:27:43 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id EE36A1F00898; Mon, 21 Sep 2026 12:27:38 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1789993662; bh=QVGKnIWRxvPTWA2suCoWlo2Ot1kTECvUDXRzzxTCpoM=; h=From:Date:Subject:References:In-Reply-To:To:Cc; b=KdhQCyIlB9toTwTgP1fKUeF0D9GY67k/HlhBgQCthL9+Bpv51wUpCNkXxQVPmnGdN Hr9YRZp2g9/na6SQKQQSQwpNDakxuzC6PvKUzI/yjkt90TBSRiae5h1hXQrLn1Sua5 UYjsQ8MLe73HFumUIWh+RPqqaL6cGzwRtneNo5/6AF2qwJMN4SOuJA9yFikStz4eFP ELYlhNbO9H+Di7qfwuRgYA05o2tCm+GSxzWqcwvoYoRxcRj70c6Mt9JAFy3AioJ7Mf 0MykNXAR/pDFVH2l+LQNtWgUouIC/bW4MAVtmKxnupn3grr2nRE7IR9n/xidfBqFuw VbQ1rDWaHluVA== From: "GUO Ren (XuanTie)" Date: Mon, 21 Sep 2026 12:27:04 +0000 Subject: [PATCH RFC v4 4/4] RISC-V: paravirt: Support nopvspin to disable PARAVIRT_SPINLOCKS MIME-Version: 1.0 Message-Id: <20260921-pvqspinlock-v4-4-409a22aed6ef@kernel.org> References: <20260921-pvqspinlock-v4-0-409a22aed6ef@kernel.org> In-Reply-To: <20260921-pvqspinlock-v4-0-409a22aed6ef@kernel.org> To: Anup Patel , Atish Patra , Paul Walmsley , Palmer Dabbelt , Albert Ou , Alexandre Ghiti , Jonathan Corbet , Shuah Khan , Randy Dunlap , Fangyu Yu , CHEN Jiankang Cc: kvm@vger.kernel.org, kvm-riscv@lists.infradead.org, linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, "GUO Ren (XuanTie)" X-Mailer: b4 0.16.0 X-BeenThere: linux-riscv@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-riscv" Errors-To: linux-riscv-bounces+linux-riscv=archiver.kernel.org@lists.infradead.org VM guests should fall back to a Test-and-Set spinlock when PARAVIRT_SPINLOCKS is disabled, because fair locks suffer from severe lock-holder preemption issues. The virt_spin_lock_key shortcuts queued_spin_lock_slowpath(), allowing virt_spin_lock() to hijack it. See commit 43b3f02899f7 ("locking/qspinlock/x86: Fix performance regression under unaccelerated VMs"). Add a static key controlling whether virt_spin_lock() is called, and add nopvspin support mirroring x86. Signed-off-by: GUO Ren (XuanTie) --- Documentation/admin-guide/kernel-parameters.txt | 2 +- arch/riscv/include/asm/qspinlock.h | 24 ++++++++++++++++++++++++ arch/riscv/kernel/qspinlock_paravirt.c | 8 ++++++++ 3 files changed, 33 insertions(+), 1 deletion(-) diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt index 33cd30996e47..d32bb7db9f91 100644 --- a/Documentation/admin-guide/kernel-parameters.txt +++ b/Documentation/admin-guide/kernel-parameters.txt @@ -4632,7 +4632,7 @@ Kernel parameters as generic guest with no PV drivers. Currently support XEN HVM, KVM, HYPER_V and VMWARE guest. - nopvspin [X86,XEN,KVM,EARLY] + nopvspin [X86,RISCV,XEN,KVM,EARLY] Disables the qspinlock slow path using PV optimizations which allow the hypervisor to 'idle' the guest on lock contention. diff --git a/arch/riscv/include/asm/qspinlock.h b/arch/riscv/include/asm/qspinlock.h index 330b714edc44..9c7108baa40a 100644 --- a/arch/riscv/include/asm/qspinlock.h +++ b/arch/riscv/include/asm/qspinlock.h @@ -12,6 +12,8 @@ /* How long a lock should spin before we consider blocking */ #define SPIN_THRESHOLD (1 << 15) +extern bool nopvspin; + void native_queued_spin_lock_slowpath(struct qspinlock *lock, u32 val); void __pv_init_lock_hash(void); void __pv_queued_spin_lock_slowpath(struct qspinlock *lock, u32 val); @@ -29,5 +31,27 @@ static inline void queued_spin_unlock(struct qspinlock *lock) #endif /* CONFIG_PARAVIRT_SPINLOCKS */ #include +#include + +/* + * KVM guests fall back to a Test-and-Set spinlock because fair locks suffer + * from severe lock-holder-preemption issues. When virt_spin_lock_key is + * enabled, virt_spin_lock() shortcuts queued_spin_lock_slowpath() and hijacks + * the lock acquisition. + */ +DECLARE_STATIC_KEY_FALSE(virt_spin_lock_key); + +#define virt_spin_lock rv_virt_spin_lock +static inline bool rv_virt_spin_lock(struct qspinlock *lock) +{ + if (!static_branch_likely(&virt_spin_lock_key)) + return false; + + do { + smp_cond_load_relaxed((s32 *)&lock->val, VAL == 0); + } while (atomic_cmpxchg(&lock->val, 0, _Q_LOCKED_VAL) != 0); + + return true; +} #endif /* _ASM_RISCV_QSPINLOCK_H */ diff --git a/arch/riscv/kernel/qspinlock_paravirt.c b/arch/riscv/kernel/qspinlock_paravirt.c index 04b13994e971..28c6c78d7e84 100644 --- a/arch/riscv/kernel/qspinlock_paravirt.c +++ b/arch/riscv/kernel/qspinlock_paravirt.c @@ -50,6 +50,8 @@ EXPORT_STATIC_CALL(pv_queued_spin_lock_slowpath); DEFINE_STATIC_CALL(pv_queued_spin_unlock, native_queued_spin_unlock); EXPORT_STATIC_CALL(pv_queued_spin_unlock); +DEFINE_STATIC_KEY_FALSE(virt_spin_lock_key); + bool __init pv_qspinlock_init(void) { if (num_possible_cpus() == 1) @@ -58,6 +60,12 @@ bool __init pv_qspinlock_init(void) if (!sbi_probe_extension(SBI_EXT_PVLOCK)) return false; + if (nopvspin) { + static_branch_enable(&virt_spin_lock_key); + pr_info("virt_spin_lock enabled by nopvspin\n"); + return true; + } + pr_info("PV qspinlocks enabled\n"); __pv_init_lock_hash(); -- 2.43.0 _______________________________________________ linux-riscv mailing list linux-riscv@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-riscv