From nobody Sat Feb 7 08:07:30 2026 Received: from mail-wr1-f73.google.com (mail-wr1-f73.google.com [209.85.221.73]) (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 0860627E7D2 for ; Tue, 24 Jun 2025 09:23:13 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.221.73 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1750756994; cv=none; b=oKwaqA/hF5DHR4ZKZndjblqpWHvXRs0d2S3Q/eAatRr7b++ZxSGvXclLByFsFeGBhBWCI5T8RDbZ0pq/QDXIup6o4APB8zBukkpyjQzrVyf/zQl2QJ6IveAh0LZzP9qdUeIk/orvWxnO6YFCd0j7DxzuF8Kh9MVijKKdO174siw= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1750756994; c=relaxed/simple; bh=sNqOydOUdFsJOifTupwq4nSErJexqWdbFSAp0X/BHs8=; h=Date:In-Reply-To:Mime-Version:References:Message-ID:Subject:From: To:Cc:Content-Type; b=Pe/JhfO3bw4tAgBsaw9/ofHjMZGolche2zUsmN7BJfXbqsoNgCk3BLi7wy1wTMpA96eQaMv1cDDqrjntZOIogFy/ekfj6ASQZjH8u19D/WHwpCvndM+NpUpk5+zKjCPlvrCWbEjmwZ43QhQ3h/khYi1T2I0ajrrs3PWStleebNU= 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=h9led1XW; arc=none smtp.client-ip=209.85.221.73 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="h9led1XW" Received: by mail-wr1-f73.google.com with SMTP id ffacd0b85a97d-3a4f8fd1847so89863f8f.1 for ; Tue, 24 Jun 2025 02:23:12 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20230601; t=1750756991; x=1751361791; 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=uwYf71jp03MdxLBkkbihaPjia5WcEiabBGw3WgO3Tes=; b=h9led1XWTZfvwQ0HGJ1WTjxeK/BJXJWnRHGOckpcDyaEjH8dNLW2GZorHYpUGrH/zi z5bXxjhy8uVdYcYTxbKm3mvRLgueza4siPq16u+syIuf5Ck6cft+EULyr12ig+cJEnL8 eCoVMLaD6ectFxpzu9H8tyyxznUA6W58dWhqz7lBk4Zo3PFZzIVDCtrGkWcPv9K40ukY z69tA0PM/ksXw3fCoL7AZeGvjD62OZXL4+MhO3lXKdrTf8hAwnrz4Wo3YKBEbVPDJLDn UFspZcgkrd9blPp3EbNNsRFmv0MKnEG/jpfLz/YF9iMmbgFIXBI3aCnJP8ey8vyJg+r0 xZtw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1750756991; x=1751361791; 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=uwYf71jp03MdxLBkkbihaPjia5WcEiabBGw3WgO3Tes=; b=oaRPpyQtdpHkZ9DCWWmadRjjihFZXS/YIDlbrChmFtPqb8DZ0yQ1Lr/3q1BNNEOjlV SSib/chMbikCud9ml2cdPnqwgrnXcfNbibHaO6WwzPh2vfq/p6/HK2YfaVrK0TRT6jCw 1fnMHK82uYaWsGLKUQ15O2DwoNo3tznB+mKnUNVjMqVrNQremcuwRDKko36AjKjdle37 eADySl04KXtGbzsbxy9gGys0hAUCHpmiyTF67TxdKqEKqYMjGvmnfUmtw1yK1UFvAyq/ pc3RdKEIAALQNccJerJ+GrZUKta4QdXjC0ofJ3NRv6oyAOZ8ndJSskOnjzUlNbvWphoX 0nGA== X-Forwarded-Encrypted: i=1; AJvYcCWsUJdZJMuwe1sp8uTzNXL/oMDi8ugLEvlyi42KjBPplOE9ao7fK+8wfZY1AgYmfR+B9/QDz+y50/Rj314=@vger.kernel.org X-Gm-Message-State: AOJu0YzeJHk8T8+3/rnaRKWA7I+gJcBBjDrCBv6l1AOIqTWqZiDD/c2u ZSjr/QF4ZlQFiudEK4DDqwVGENoKmWbvnJGb3MC2t7wzxnRJIchHd56668P119U3bmjrZMmlysB RKA== X-Google-Smtp-Source: AGHT+IH24B8WCr6OqpYTzgXlBOmAaqywmufYf+A6d0IPqpHPXgTUr1i0hczj9/ID0CWcv/+olmz2NYEObg== X-Received: from wmqd12.prod.google.com ([2002:a05:600c:34cc:b0:450:cb8f:62cc]) (user=keirf job=prod-delivery.src-stubby-dispatcher) by 2002:a05:6000:2004:b0:3a4:f50b:ca2 with SMTP id ffacd0b85a97d-3a6d12fae98mr11445680f8f.8.1750756991731; Tue, 24 Jun 2025 02:23:11 -0700 (PDT) Date: Tue, 24 Jun 2025 09:22:55 +0000 In-Reply-To: <20250624092256.1105524-1-keirf@google.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20250624092256.1105524-1-keirf@google.com> X-Mailer: git-send-email 2.50.0.rc2.761.g2dc52ea45b-goog Message-ID: <20250624092256.1105524-4-keirf@google.com> Subject: [PATCH 3/3] 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: 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 3bde4fb5c6aa..28a63f1ad314 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 eec82775c5bf..b7d4da8ba0b2 100644 --- a/virt/kvm/kvm_main.c +++ b/virt/kvm/kvm_main.c @@ -5924,6 +5924,13 @@ 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) { @@ -5962,8 +5969,7 @@ 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.rc2.761.g2dc52ea45b-goog