From nobody Mon Feb 9 08:53:52 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 C203131B12D for ; Tue, 19 Aug 2025 09:09:03 +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=1755594545; cv=none; b=Ob2g8Jsp4eMPiaf7+oeqW8yzshC0TgEMCKhVfLIzbpO4840Zfq4jngQnHTBOGwhuMwhCiY9D4ewSbcjCyIBZ93F2CLih93Zv7HVDi2o9MyEgC0SaxKOk4EpkzYyO6awdDoAwLOB/o1HixCiXkebW5YY+zR7If8cP0DDar72B3VQ= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1755594545; c=relaxed/simple; bh=H2tOAJiT9+UKslPsSm2DpbqQjItGsMTWZFtsWcCliJo=; h=Date:In-Reply-To:Mime-Version:References:Message-ID:Subject:From: To:Cc:Content-Type; b=NNneBp7ZFZsMqklF562b+7PJ+skc4K1Bo+1Eg7GDuxvmwok0bV0Dhtj7R7UyaAktahs57vFCFc5G0UrYgejTk4iBBmpEEAr2llo8p4EafS8/Ty2aLFcfo1kPP0ndeD54CzCX305k/NAb9vWuFFj1XjgE9tqqJ/zBAuQq4156yQU= 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=fYXNToH9; 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="fYXNToH9" Received: by mail-wr1-f73.google.com with SMTP id ffacd0b85a97d-3b9d41b779aso3277409f8f.0 for ; Tue, 19 Aug 2025 02:09:03 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20230601; t=1755594542; x=1756199342; 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=Dryunec7C15FT43WsTYqyK8BC2tH9vRzD5ncvT1VTyI=; b=fYXNToH9tEqgbEMXVOP5e5crtcIxIxr+Uk7zs27IWvQhA5rlV9cw2n5Gr4i1RSqO9u OSUsTNsfQw40GtiIQ8viYHgfpbsyxzt+Y4Z4OeLH4CqfSZBu6iT85sZ3N+lIfq0nJsmt OSHbojn6RtuhFGZrgsgTRoP5qrfN4Yu0TTnvhF+DHThf/qKRMmgbo/CmNIGlfYXGfjb5 h7baHupfizv+ggwN787/wgd9mvqjLHrFrLc530OfPDPrg/iseoFcii4f2xX4/PyBk8+I q18KOIViTOhkj61WHgqWOgyt3cxVfK/lqmXeQlZaGUNbpunOpwzodlVsbG/detztO2GL gObg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1755594542; x=1756199342; 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=Dryunec7C15FT43WsTYqyK8BC2tH9vRzD5ncvT1VTyI=; b=NgggKVXwCWJNnBWdS2Xv/nIAxIuvzuukJ2yyBnKL42DxxEZefYY2NMiSPezpA5Okwz cEY8UGyKngrWJBACt3uWCb5BoSC3Y54v2BdEIJL1HpsMjL/06AGM8e0sF57GZZj5/yP4 6CXIANY7HOnuDcHGIwaP1FtUV8H82j4ncCX8EqiaARTsEkGlj/y5pgrBP/VJdXnpP0Ak kDVdA27pLPsw77WicsM6Ywua6ETRw4SJCxEHHwiNwmP2DNI1BLB4ggaVjfKXlb3EIwnt Z2vLSs3zLEIHsEWdP3GmLYBe5jazvkzfBH27FXnoAsJ6giiEa2gXJ7yTGg84C5lNOD9h DyHg== X-Forwarded-Encrypted: i=1; AJvYcCVteIJWWkX704Dz03GmyH+TOCO6MPxnh+GEx3Q+RwmjDlHrIKSGxP3ePfQWnGzDsEasLzKH5Fik8fwQHF0=@vger.kernel.org X-Gm-Message-State: AOJu0YwfpbTsoFHY/aQa/UG6x/+8GTsIeuK61fDhATDPyWiRIMg/gssO ON9H24kv8K3DboKBCAQADGSBlWHm51cpFED7C4bPGrj9HsA+fX9f0h64bMaxBQ6qCw8v55vRkTZ vpg== X-Google-Smtp-Source: AGHT+IEWZK/S3bmfcTBpMD/OWSO9i/fgVmwIt0LtH8csguKl7P4ZrXGSAdstuEmNYdGpRILLFl16F7mudg== X-Received: from wrqx7.prod.google.com ([2002:a5d:4907:0:b0:3b7:8342:dc50]) (user=keirf job=prod-delivery.src-stubby-dispatcher) by 2002:a5d:64ef:0:b0:3b7:9c28:f846 with SMTP id ffacd0b85a97d-3c0ed1f3372mr1452415f8f.44.1755594542204; Tue, 19 Aug 2025 02:09:02 -0700 (PDT) Date: Tue, 19 Aug 2025 09:08:53 +0000 In-Reply-To: <20250819090853.3988626-1-keirf@google.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20250819090853.3988626-1-keirf@google.com> X-Mailer: git-send-email 2.51.0.rc1.193.gad69d77794-goog Message-ID: <20250819090853.3988626-5-keirf@google.com> Subject: [PATCH v3 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 e7d6111cf254..103be35caf0d 100644 --- a/include/linux/kvm_host.h +++ b/include/linux/kvm_host.h @@ -206,6 +206,7 @@ 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 4f35ae23ee5a..9144a0b4a268 100644 --- a/virt/kvm/kvm_main.c +++ b/virt/kvm/kvm_main.c @@ -5953,6 +5953,13 @@ 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) { @@ -5991,8 +5998,7 @@ 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 2.51.0.rc1.193.gad69d77794-goog