From nobody Fri Oct 3 21:05:41 2025 Received: from m16.mail.163.com (m16.mail.163.com [117.135.210.4]) by smtp.subspace.kernel.org (Postfix) with ESMTP id 03A931EA80; Mon, 25 Aug 2025 12:15:41 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=117.135.210.4 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1756124145; cv=none; b=pXPtTlxQWKEMF+PIwQyDL7jo58IcwJo4sI+gSk7ZrLWYpvplkqLXqtJuAichOnOiQjQ+0MTiNNsXrWCQLPHqaLwpsJLujgyJlbJCurjRW9U5PN7u/tWPPeKEVw2iLqEuzjc93DnxISQNWco8Jy9q4up6SsWIMPy+FOllFV8mYmk= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1756124145; c=relaxed/simple; bh=pQRUIrRaJ4WHpO9221p/eCIx31S0mIW0eHDvL9xlmjw=; h=From:To:Cc:Subject:Date:Message-ID:MIME-Version; b=A73sK8LA5NYFuiI5nKoU5lVJUw8Yu11Xa9MCyrCu+kDVKPvqEukKRoj1EB7e8/cZe7Tbgcs8bNhWOIcNbvnda6IasxJXoWRPv5pjAxgV1E+uav1Id85TeR2C+jYPlWf28DiouA1Z6WdkFI6+s3hf/hQRk3U8TdrArj79ugzaHgU= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=163.com; spf=pass smtp.mailfrom=163.com; dkim=pass (1024-bit key) header.d=163.com header.i=@163.com header.b=oesLLvfI; arc=none smtp.client-ip=117.135.210.4 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=163.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=163.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=163.com header.i=@163.com header.b="oesLLvfI" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=163.com; s=s110527; h=From:To:Subject:Date:Message-ID:MIME-Version; bh=wm j5n76lJI+EQ4fSCNHd5tg/1m0AUVHlQPLj3ohhhUY=; b=oesLLvfIRErVrkDogt a1OiIZOZxwCxnyD4vqmBrnVVnRE80lSQFCaARmR+gtW5WcBIqVPm0z8/9JginPJi BJSCAAwBOWQ0Aaaza4t++VbR9D/xdZ4ULSDfoDFvbUDgzCkA4qdjYYMSMerFg3Sb bq23otVjkEWrLMTDxEZQrHoFc= Received: from localhost.localdomain (unknown []) by gzga-smtp-mtada-g0-3 (Coremail) with SMTP id _____wCn0LeWU6xoYo0IEQ--.6105S2; Mon, 25 Aug 2025 20:14:15 +0800 (CST) From: Jinyu Tang To: Anup Patel , Atish Patra , Andrew Jones , Conor Dooley , Yong-Xuan Wang , Paul Walmsley , Nutty Liu Cc: kvm@vger.kernel.org, kvm-riscv@lists.infradead.org, linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org, Jinyu Tang Subject: [PATCH v3] riscv: skip csr restore if vcpu preempted reload Date: Mon, 25 Aug 2025 20:14:11 +0800 Message-ID: <20250825121411.86573-1-tjytimi@163.com> X-Mailer: git-send-email 2.43.0 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable X-CM-TRANSID: _____wCn0LeWU6xoYo0IEQ--.6105S2 X-Coremail-Antispam: 1Uf129KBjvJXoWxuryrZw13Kr13uF15tw47Jwb_yoW5Ww48pF W7urs09w48JrW7G342qrsY9F4F9rZYgrn3XryDWrWSyr1Utr9Yyr4kK3y7AFy5GryrZF1S yFyDtFyIkFnYvwUanT9S1TB71UUUUU7qnTZGkaVYY2UrUUUUjbIjqfuFe4nvWSU5nxnvy2 9KBjDUYxBIdaVFxhVjvjDU0xZFpf9x0piHa0PUUUUU= X-CM-SenderInfo: xwm13xlpl6il2tof0z/1tbipR+0eGisTjSwYwAAsb Content-Type: text/plain; charset="utf-8" The kvm_arch_vcpu_load() function is called in two cases for riscv: 1. When entering KVM_RUN from userspace ioctl. 2. When a preempted VCPU is scheduled back. In the second case, if no other KVM VCPU has run on this CPU since the current VCPU was preempted, the guest CSR (including AIA CSRS and HGTAP)=20 values are still valid in the hardware and do not need to be restored. This patch is to skip the CSR write path when: 1. The VCPU was previously preempted (vcpu->scheduled_out =3D=3D 1). 2. It is being reloaded on the same physical CPU (vcpu->arch.last_exit_cpu =3D=3D cpu). 3. No other KVM VCPU has used this CPU in the meantime (vcpu =3D=3D __this_cpu_read(kvm_former_vcpu)). This reduces many CSR writes with frequent preemption on the same CPU. Signed-off-by: Jinyu Tang Reviewed-by: Nutty Liu Reviewed-by: Andrew Jones --- v2 -> v3: v2 was missing a critical check because I generated the patch from my wrong (experimental) branch. This is fixed in v3. Sorry for my trouble. v1 -> v2: Apply the logic to aia csr load. Thanks for Andrew Jones's advice. arch/riscv/kvm/vcpu.c | 13 +++++++++++-- 1 file changed, 11 insertions(+), 2 deletions(-) diff --git a/arch/riscv/kvm/vcpu.c b/arch/riscv/kvm/vcpu.c index f001e5640..66bd3ddd5 100644 --- a/arch/riscv/kvm/vcpu.c +++ b/arch/riscv/kvm/vcpu.c @@ -25,6 +25,8 @@ #define CREATE_TRACE_POINTS #include "trace.h" =20 +static DEFINE_PER_CPU(struct kvm_vcpu *, kvm_former_vcpu); + const struct _kvm_stats_desc kvm_vcpu_stats_desc[] =3D { KVM_GENERIC_VCPU_STATS(), STATS_DESC_COUNTER(VCPU, ecall_exit_stat), @@ -581,6 +583,10 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu) struct kvm_vcpu_csr *csr =3D &vcpu->arch.guest_csr; struct kvm_vcpu_config *cfg =3D &vcpu->arch.cfg; =20 + if (vcpu->scheduled_out && vcpu =3D=3D __this_cpu_read(kvm_former_vcpu) && + vcpu->arch.last_exit_cpu =3D=3D cpu) + goto csr_restore_done; + if (kvm_riscv_nacl_sync_csr_available()) { nsh =3D nacl_shmem(); nacl_csr_write(nsh, CSR_VSSTATUS, csr->vsstatus); @@ -624,6 +630,9 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu) =20 kvm_riscv_mmu_update_hgatp(vcpu); =20 + kvm_riscv_vcpu_aia_load(vcpu, cpu); + +csr_restore_done: kvm_riscv_vcpu_timer_restore(vcpu); =20 kvm_riscv_vcpu_host_fp_save(&vcpu->arch.host_context); @@ -633,8 +642,6 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu) kvm_riscv_vcpu_guest_vector_restore(&vcpu->arch.guest_context, vcpu->arch.isa); =20 - kvm_riscv_vcpu_aia_load(vcpu, cpu); - kvm_make_request(KVM_REQ_STEAL_UPDATE, vcpu); =20 vcpu->cpu =3D cpu; @@ -645,6 +652,8 @@ void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu) void *nsh; struct kvm_vcpu_csr *csr =3D &vcpu->arch.guest_csr; =20 + __this_cpu_write(kvm_former_vcpu, vcpu); + vcpu->cpu =3D -1; =20 kvm_riscv_vcpu_aia_put(vcpu); --=20 2.43.0