From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-pg1-f202.google.com (mail-pg1-f202.google.com [209.85.215.202]) (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 98D5C28DB54 for ; Mon, 8 Sep 2025 19:44:52 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.215.202 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1757360694; cv=none; b=RaXHuefY8X3ATbHt5+5ijUVnasOpe+pCyzEc9CcaZoQhBp+1zUieENHcunTavUdPjVf8whLMhgl8L1fImn4cxrqC1OWn1zHVmlYqCoiXL/de0zO28OAH2XjVxLj4k50NSwFKkHdK77DiFX5S45D/AB+qK9WdbnCRBoaUJK2jixw= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1757360694; c=relaxed/simple; bh=xxX6qsulkVirIUPMtxxcMy/WlTsw7wbx1cEPYNOUvQ4=; h=Date:In-Reply-To:Mime-Version:References:Message-ID:Subject:From: To:Cc:Content-Type; b=OYmNQgk0VcRlwFrBeMK32dyikXW/nL9w7VVjfKaOZpPWV1Dz5lhQL3eyeZSRqBZVseDangCwElWHm2u4HuTBhCEAq16Y8d/cCDbR8ih2P0rxEVZHI9BcTx5d0lCPuwzPoWcZwVT/IaZwp6tvLplf8owVl4dAYQ6u7uIwgMAQQUw= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=google.com; spf=pass smtp.mailfrom=flex--seanjc.bounces.google.com; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b=uO2ir1uH; arc=none smtp.client-ip=209.85.215.202 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--seanjc.bounces.google.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="uO2ir1uH" Received: by mail-pg1-f202.google.com with SMTP id 41be03b00d2f7-b4c949fc524so3736983a12.2 for ; Mon, 08 Sep 2025 12:44:52 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20230601; t=1757360692; x=1757965492; 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=+BN9f9TRNhPzZGbZcN/O3Z0X+3jX9IGhyykVuk72IV0=; b=uO2ir1uHTtgukzGG0spIgfg73ve9aliGXE89LTjOEkzUnWj/hTErCIvAwFr+M6iZwp 67gTNZ7b5OA1pa5LjnHBZz0E4YylwDGqL+yFJBQuonphkgqKxrUfSd3Uc2HqnSCBpI5S MVHJFjlraIreUeahQLMom+9wo8msLioW/kVGzsjyd2uuf0tVj3a9pF5BSRRUALaUe+TD +MXJ9X9NEeVMrRNdykWQLFfDdx6negXe+fNELJaiNW/xZN3i48pB2fWPY2u8uOeLjyoi WiUUvDGVritslS0J5MndrANAaep1frTiTnCjmC/OaKTzkBZZQMhEx81BITkVmVwXRlIE cbgQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1757360692; x=1757965492; 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=+BN9f9TRNhPzZGbZcN/O3Z0X+3jX9IGhyykVuk72IV0=; b=nwZuln1m+cpUwEUfbigVQ+IVXr/y0V0VRJCwwX6w1clrm1O9wUb7yELjy2mA+o8hG4 1otlToWmW85mOnj3Gm0VwJwnwEYdqyZsOp9pygJmp1UGbSmYPsg2EgE7sPuOToqrcfdy XHZrbcIhqr5hrgeTQFRQ2uar+kJOsnSHxPU6bfD3mlrFPICeBGyH285oUU/TcVHdJTBX i8Lt/fpoplyHlFCMGVtaYG5hrAAAZH3HjhQQdHy61/o9abjFXIWH+3I3rfQcK42bb76o s0dJPL4yEgxvpt2mlpTESQDMXV7zziopnvWPTj8fpqQ2MAvSg+uKnqacp4bwPPvI7VkK +gZA== X-Forwarded-Encrypted: i=1; AJvYcCVvIEN8yUxrsXtCAVYxOkdHs0p6fX3v7TeEy60RQtGCVQ2oFaJ8Rd9n5fDsGhqW7OrnuVk=@vger.kernel.org X-Gm-Message-State: AOJu0Yz/w+JflsnjbbOFbdqhiWhwUswdDgw80qd9bvsswmTexBxResZT seCcu2dRsjmI8oxvzRWaiEVBr5r4HiLFHfIzoRjqnKTPWTS0u+YDV01/cmuetLSAisnew1HQoMv XeTF0GA== X-Google-Smtp-Source: AGHT+IFxCnWEFD2vXNWgOPEAttN5OOtAqn765A51cfCV4FQPi8e13adf6at6Ei8q1uZLTqO9SeFgLpD0kRY= X-Received: from pjbsq16.prod.google.com ([2002:a17:90b:5310:b0:329:ccdd:e725]) (user=seanjc job=prod-delivery.src-stubby-dispatcher) by 2002:a17:90b:1dc4:b0:327:e9f4:4dd8 with SMTP id 98e67ed59e1d1-32d43f2f4b6mr13695527a91.10.1757360691927; Mon, 08 Sep 2025 12:44:51 -0700 (PDT) Date: Mon, 8 Sep 2025 12:44:50 -0700 In-Reply-To: <20250819090853.3988626-5-keirf@google.com> Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20250819090853.3988626-1-keirf@google.com> <20250819090853.3988626-5-keirf@google.com> Message-ID: Subject: Re: [PATCH v3 4/4] KVM: Avoid synchronize_srcu() in kvm_io_bus_register_dev() From: Sean Christopherson To: Keir Fraser Cc: linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, kvm@vger.kernel.org, Eric Auger , Oliver Upton , Marc Zyngier , Will Deacon , Paolo Bonzini Content-Type: text/plain; charset="us-ascii" On Tue, Aug 19, 2025, Keir Fraser wrote: > 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[]; > }; > > 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); > > +static void __free_bus(struct rcu_head *rcu) > +{ > + struct kvm_io_bus *bus = 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 addr, > 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); To address the syzkaller splat, KVM needs to call srcu_barrier() prior to freeing the structure. diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c index 9144a0b4a268..62693f18ecf4 100644 --- a/virt/kvm/kvm_main.c +++ b/virt/kvm/kvm_main.c @@ -1321,6 +1321,7 @@ static void kvm_destroy_vm(struct kvm *kvm) kvm_free_memslots(kvm, &kvm->__memslots[i][1]); } cleanup_srcu_struct(&kvm->irq_srcu); + srcu_barrier(&kvm->srcu); cleanup_srcu_struct(&kvm->srcu); #ifdef CONFIG_KVM_GENERIC_MEMORY_ATTRIBUTES xa_destroy(&kvm->mem_attr_array);