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dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:58554 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1kX07h-0003oi-8E for qemu-devel@archiver.kernel.org; Mon, 26 Oct 2020 06:53:09 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:47452) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1kWzyU-00017X-Iq for qemu-devel@nongnu.org; Mon, 26 Oct 2020 06:43:38 -0400 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:31841) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_CBC_SHA1:256) (Exim 4.90_1) (envelope-from ) id 1kWzyQ-00038B-J9 for qemu-devel@nongnu.org; Mon, 26 Oct 2020 06:43:37 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1603709012; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=HnRu1imVa6WrgcuXb3u0XjybwlD2HwewTklUrOUp13A=; b=QGFH2tdlmkStLA5gV7QuoHO7LLaUQdM0eZqNkpuU9SI3t+xvx3ukc2olHgUT7ifdR+9r6v cK1ZepggpctO0FiIH/bWs86M5TC1lKxextr5uGFKfkWF0zQ/3JFw13/0Ko/pAmjftbcAdt 4/KvWCU62cfqNGqDyQqkd8zJVQt2ArQ= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-445-O8a212Z8PtSyAyLSsOlypw-1; Mon, 26 Oct 2020 06:43:29 -0400 X-MC-Unique: O8a212Z8PtSyAyLSsOlypw-1 Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.phx2.redhat.com [10.5.11.15]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id A4CB361182 for ; Mon, 26 Oct 2020 10:43:28 +0000 (UTC) Received: from [10.36.113.62] (ovpn-113-62.ams2.redhat.com [10.36.113.62]) by smtp.corp.redhat.com (Postfix) with ESMTP id C113875124; Mon, 26 Oct 2020 10:43:23 +0000 (UTC) Subject: Re: [PATCH RFC] memory: pause all vCPUs for the duration of memory transactions To: Vitaly Kuznetsov , qemu-devel@nongnu.org References: <20201026084916.3103221-1-vkuznets@redhat.com> From: David Hildenbrand Organization: Red Hat GmbH Message-ID: <294a987d-b0ef-1b58-98ac-0d4d43075d6e@redhat.com> Date: Mon, 26 Oct 2020 11:43:22 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.3.1 MIME-Version: 1.0 In-Reply-To: <20201026084916.3103221-1-vkuznets@redhat.com> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.15 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=david@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Received-SPF: pass client-ip=216.205.24.124; envelope-from=david@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/10/26 02:39:09 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, NICE_REPLY_A=-0.001, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H4=-0.01, RCVD_IN_MSPIKE_WL=-0.01, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Paolo Bonzini , Laszlo Ersek , Eduardo Habkost , Peter Xu , "Dr. David Alan Gilbert" Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 26.10.20 09:49, Vitaly Kuznetsov wrote: > Currently, KVM doesn't provide an API to make atomic updates to memmap when > the change touches more than one memory slot, e.g. in case we'd like to > punch a hole in an existing slot. > > Reports are that multi-CPU Q35 VMs booted with OVMF sometimes print something > like > > !!!! X64 Exception Type - 0E(#PF - Page-Fault) CPU Apic ID - 00000003 !!!! > ExceptionData - 0000000000000010 I:1 R:0 U:0 W:0 P:0 PK:0 SS:0 SGX:0 > RIP - 000000007E35FAB6, CS - 0000000000000038, RFLAGS - 0000000000010006 > RAX - 0000000000000000, RCX - 000000007E3598F2, RDX - 00000000078BFBFF > ... > > The problem seems to be that TSEG manipulations on one vCPU are not atomic > from other vCPUs views. In particular, here's the strace: > > Initial creation of the 'problematic' slot: > > 10085 ioctl(13, KVM_SET_USER_MEMORY_REGION, {slot=6, flags=0, guest_phys_addr=0x100000, > memory_size=2146435072, userspace_addr=0x7fb89bf00000}) = 0 > > ... and then the update (caused by e.g. mch_update_smram()) later: > > 10090 ioctl(13, KVM_SET_USER_MEMORY_REGION, {slot=6, flags=0, guest_phys_addr=0x100000, > memory_size=0, userspace_addr=0x7fb89bf00000}) = 0 > 10090 ioctl(13, KVM_SET_USER_MEMORY_REGION, {slot=6, flags=0, guest_phys_addr=0x100000, > memory_size=2129657856, userspace_addr=0x7fb89bf00000}) = 0 > > In case KVM has to handle any event on a different vCPU in between these > two calls the #PF will get triggered. > > An ideal solution to the problem would probably require KVM to provide a > new API to do the whole transaction in one shot but as a band-aid we can > just pause all vCPUs to make memory transations atomic. > > Reported-by: Dr. David Alan Gilbert > Signed-off-by: Vitaly Kuznetsov > --- > RFC: Generally, memap updates happen only a few times during guest boot but > I'm not sure there are no scenarios when pausing all vCPUs is undesireable > from performance point of view. Also, I'm not sure if kvm_enabled() check > is needed. > Signed-off-by: Vitaly Kuznetsov > --- > softmmu/memory.c | 11 +++++++++-- > 1 file changed, 9 insertions(+), 2 deletions(-) > > diff --git a/softmmu/memory.c b/softmmu/memory.c > index fa280a19f7f7..0bf6f3f6d5dc 100644 > --- a/softmmu/memory.c > +++ b/softmmu/memory.c > @@ -28,6 +28,7 @@ > > #include "exec/memory-internal.h" > #include "exec/ram_addr.h" > +#include "sysemu/cpus.h" > #include "sysemu/kvm.h" > #include "sysemu/runstate.h" > #include "sysemu/tcg.h" > @@ -1057,7 +1058,9 @@ static void address_space_update_topology(AddressSpace *as) > void memory_region_transaction_begin(void) > { > qemu_flush_coalesced_mmio_buffer(); > - ++memory_region_transaction_depth; > + if ((++memory_region_transaction_depth == 1) && kvm_enabled()) { > + pause_all_vcpus(); > + } > } > > void memory_region_transaction_commit(void) > @@ -1087,7 +1090,11 @@ void memory_region_transaction_commit(void) > } > ioeventfd_update_pending = false; > } > - } > + > + if (kvm_enabled()) { > + resume_all_vcpus(); > + } > + } > } > > static void memory_region_destructor_none(MemoryRegion *mr) > This is in general unsafe. pause_all_vcpus() will temporarily drop the BQL, resulting in bad things happening to caller sites. I studies the involved issues quite intensively when wanting to resize memory regions from virtio-mem code. It's not that easy. Have a look at my RFC for resizing. You can apply something similar to other operations. https://www.mail-archive.com/qemu-devel@nongnu.org/msg684979.html -- Thanks, David / dhildenb