From nobody Mon Feb 9 06:24:54 2026 Received: from mail-wr1-f74.google.com (mail-wr1-f74.google.com [209.85.221.74]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 21B142F433C for ; Wed, 16 Jul 2025 11:07:50 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.221.74 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1752664071; cv=none; b=GMB6dmd8GuglslXP87LVB6FPHXzfgDpGI8kmYVBxpKhD5hLINp7QfMy9LkrrZBZTr3J9whwO23YzErDoWNGkWeX6cVB6rgERtwbypfQhlRoTUuzVLWL5xiGSXAwt9TMrNCKrAUHequ7TyR7V8nRLmXqy1bmBUHly0MwyKe5UwH8= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1752664071; c=relaxed/simple; bh=SwRbVQZAJVuZZ3KPArKlcrn8a8LengWToebo4ivdzPo=; h=Date:In-Reply-To:Mime-Version:References:Message-ID:Subject:From: To:Cc:Content-Type; b=fwbbg45qzUfW80scpyQwRI5u5KGnnP3veFRcSZ1a0aOvrK2fTcMu1Ajj2sKhe/wZ2QxN7PhmrXH483kK9pj7Cso0lc3kLQ2PamA4q+FJfF+3dqghccSViO+tdrlXerTotQUPQHEHfI39Po/V5cdZ9q28kUHsrBaLEpsRcJLMbM4= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=google.com; spf=pass smtp.mailfrom=flex--keirf.bounces.google.com; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b=3ZTgm0gA; arc=none smtp.client-ip=209.85.221.74 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=google.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=flex--keirf.bounces.google.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="3ZTgm0gA" Received: by mail-wr1-f74.google.com with SMTP id ffacd0b85a97d-3a52bfda108so3166697f8f.3 for ; Wed, 16 Jul 2025 04:07:50 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20230601; t=1752664069; x=1753268869; darn=vger.kernel.org; h=cc:to:from:subject:message-id:references:mime-version:in-reply-to :date:from:to:cc:subject:date:message-id:reply-to; bh=bFiBh24vfyxUkvL/qAF91ZLaWMijXEEuq00t+Y6HBLE=; b=3ZTgm0gAiZmMJU+oBt2B575JcO1wGtvdpQ9FrcIeml0lmRslMfAPKZGeL9D9NoL8pZ rPhFECcZ98Wdu8mk0a6KfRsvGYPlPmmi0jBK/b8/vaZOiolxrz1Xj4bLftBszW5HwYCW Rj8eqQOm2XuCtDDy51GdpCg9EmgAjrhUfw/KxRiDbjTZ7A595qLGfQTHH0dD6hxtf/BG eyeEImos8O5UZ4YyCGJs9u7D6Xhq835577Vq7ZeJbDrm3DG4dXCKqJEs6fUcLa1SICYE gy8iTgy4InENVuloePP9+8MTT8VVsLoLGeTzcpOcmCCDoqpF/CcuJRL01RfTCXGtzvr9 IZHw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1752664069; x=1753268869; h=cc:to:from:subject:message-id:references:mime-version:in-reply-to :date:x-gm-message-state:from:to:cc:subject:date:message-id:reply-to; bh=bFiBh24vfyxUkvL/qAF91ZLaWMijXEEuq00t+Y6HBLE=; b=ix0PAn1a6RRt/ayw0KrZI9JYfVjwzSCaj0e7DbINA5mKD403iHpVH0fLKiBFegIgOU WBWkKZRIKBpLZ0gswzGQdi7Db1dRO2vGrbCg7CAEkYdn+N0AahBKUVWBzeUzz/gw9Fbh 9vScxY15+9NtfzP+KHY72/lgddstjFWyAMsVRvF4z7qHNHzyk0vPtCx27eGIvd5eSJLr L4Uf4Z/vuAn8J8YGvS3nC7eA+AEgcR8GmrFYrjCuMp56+LsAK9Gc8K7b1Eh7rGaSE2wr LVkhQVJgPKsCHV/N49KKxV+EMg2ki2BZu+KL0/vPl5Ta/gpHJ4zFoWRmS0cOLc3TL6Uo kJdw== X-Forwarded-Encrypted: i=1; AJvYcCXZcdVgYhR0ftqNFoLSjbGMPUaoeloxKIin2645FaMhfPQYR2bIay/HHqUFHj7ECSx+q9RgLWlsiy2O5aM=@vger.kernel.org X-Gm-Message-State: AOJu0Yye+FN3KD+aDDPPamPt+bEyNJW9N1MVWxcZNVX0pDbz0Sh/gMwk udor18O55E2f47T7mIrfsRvnJzHDli9t6KR6XKBUHCDQ2bAwm7y2dF/hF4co7iVIwgUL77LksU+ rwg== X-Google-Smtp-Source: AGHT+IHS3mDNKI5f7HxVRvsGrhgPtKyzq6M9m0VkLvornMAR5mJ3Yg4oC8wem/+qjRBiPoyYxBKnOYGMMA== X-Received: from wmqd13.prod.google.com ([2002:a05:600c:34cd:b0:456:23aa:8bf]) (user=keirf job=prod-delivery.src-stubby-dispatcher) by 2002:a05:6000:2486:b0:3a5:232c:6976 with SMTP id ffacd0b85a97d-3b60dd99677mr1925159f8f.44.1752664068593; Wed, 16 Jul 2025 04:07:48 -0700 (PDT) Date: Wed, 16 Jul 2025 11:07:37 +0000 In-Reply-To: <20250716110737.2513665-1-keirf@google.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20250716110737.2513665-1-keirf@google.com> X-Mailer: git-send-email 2.50.0.727.gbf7dc18ff4-goog Message-ID: <20250716110737.2513665-5-keirf@google.com> Subject: [PATCH v2 4/4] KVM: Avoid synchronize_srcu() in kvm_io_bus_register_dev() From: Keir Fraser To: linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, kvm@vger.kernel.org Cc: Sean Christopherson , Eric Auger , Oliver Upton , Marc Zyngier , Will Deacon , Paolo Bonzini , Keir Fraser Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" Device MMIO registration may happen quite frequently during VM boot, and the SRCU synchronization each time has a measurable effect on VM startup time. In our experiments it can account for around 25% of a VM's startup time. Replace the synchronization with a deferred free of the old kvm_io_bus structure. Signed-off-by: Keir Fraser --- include/linux/kvm_host.h | 1 + virt/kvm/kvm_main.c | 10 ++++++++-- 2 files changed, 9 insertions(+), 2 deletions(-) diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h index 9132148fb467..802ca46f7537 100644 --- a/include/linux/kvm_host.h +++ b/include/linux/kvm_host.h @@ -205,6 +205,7 @@ struct kvm_io_range { struct kvm_io_bus { int dev_count; int ioeventfd_count; + struct rcu_head rcu; struct kvm_io_range range[]; }; =20 diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c index 9ec3b96b9666..f690a4997a0f 100644 --- a/virt/kvm/kvm_main.c +++ b/virt/kvm/kvm_main.c @@ -5948,6 +5948,13 @@ int kvm_io_bus_read(struct kvm_vcpu *vcpu, enum kvm_= bus bus_idx, gpa_t addr, } EXPORT_SYMBOL_GPL(kvm_io_bus_read); =20 +static void __free_bus(struct rcu_head *rcu) +{ + struct kvm_io_bus *bus =3D container_of(rcu, struct kvm_io_bus, rcu); + + kfree(bus); +} + int kvm_io_bus_register_dev(struct kvm *kvm, enum kvm_bus bus_idx, gpa_t a= ddr, int len, struct kvm_io_device *dev) { @@ -5986,8 +5993,7 @@ int kvm_io_bus_register_dev(struct kvm *kvm, enum kvm= _bus bus_idx, gpa_t addr, memcpy(new_bus->range + i + 1, bus->range + i, (bus->dev_count - i) * sizeof(struct kvm_io_range)); rcu_assign_pointer(kvm->buses[bus_idx], new_bus); - synchronize_srcu_expedited(&kvm->srcu); - kfree(bus); + call_srcu(&kvm->srcu, &bus->rcu, __free_bus); =20 return 0; } --=20 2.50.0.727.gbf7dc18ff4-goog