From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 265E7C54FB3 for ; Mon, 26 May 2025 09:17:35 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1uJTxB-0007hU-Gu; Mon, 26 May 2025 05:17:05 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1uJTwq-0007ao-4Q for qemu-devel@nongnu.org; Mon, 26 May 2025 05:16:49 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.129.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1uJTwo-0002B3-0p for qemu-devel@nongnu.org; Mon, 26 May 2025 05:16:43 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1748251000; 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=9/nly/qMJ+0+UJI4bd1qWVRcyddHc8dlfjTlrYVD9Bw=; b=Z2f7mUgYBsQ7GK6OhtHiW9uVe0bbZ3gGcwUuohmtTfk36zA801HguEAciFn3ATMRvZM2eT izKTKto5s7tsGTmthcVadoV9pGofoEhirRVW5oOT5T63cDTp2X3+nEkzCAYWcMPbJZ8ecK nndveWkJB8MnyhOsRN+g0j/gP0kEkHE= Received: from mx-prod-mc-08.mail-002.prod.us-west-2.aws.redhat.com (ec2-35-165-154-97.us-west-2.compute.amazonaws.com [35.165.154.97]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-59-qOnVj3BVMo-J-1fGJBbLJg-1; Mon, 26 May 2025 05:16:36 -0400 X-MC-Unique: qOnVj3BVMo-J-1fGJBbLJg-1 X-Mimecast-MFC-AGG-ID: qOnVj3BVMo-J-1fGJBbLJg_1748250994 Received: from mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.111]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-08.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 9EB55180048E; Mon, 26 May 2025 09:16:34 +0000 (UTC) Received: from blackfin.pond.sub.org (unknown [10.45.242.2]) by mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 3E81618003FD; Mon, 26 May 2025 09:16:34 +0000 (UTC) Received: by blackfin.pond.sub.org (Postfix, from userid 1000) id B375921E66C3; Mon, 26 May 2025 11:16:31 +0200 (CEST) From: Markus Armbruster To: Si-Wei Liu Cc: Jason Wang , Jonah Palmer , qemu-devel@nongnu.org, eperezma@redhat.com, peterx@redhat.com, mst@redhat.com, lvivier@redhat.com, dtatulea@nvidia.com, leiyang@redhat.com, parav@mellanox.com, sgarzare@redhat.com, lingshan.zhu@intel.com, boris.ostrovsky@oracle.com Subject: Re: [PATCH v4 0/7] Move memory listener register to vhost_vdpa_init In-Reply-To: <50a648fa-76ab-47bf-9f6e-c07da913cb52@oracle.com> (Si-Wei Liu's message of "Fri, 16 May 2025 12:09:42 -0700") References: <20250507184647.15580-1-jonah.palmer@oracle.com> <87plg9ukgq.fsf@pond.sub.org> <50a648fa-76ab-47bf-9f6e-c07da913cb52@oracle.com> Date: Mon, 26 May 2025 11:16:31 +0200 Message-ID: <87frgr7mvk.fsf@pond.sub.org> User-Agent: Gnus/5.13 (Gnus v5.13) MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.111 Received-SPF: pass client-ip=170.10.129.124; envelope-from=armbru@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -59 X-Spam_score: -6.0 X-Spam_bar: ------ X-Spam_report: (-6.0 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-2.904, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H5=-1, RCVD_IN_MSPIKE_WL=-0.01, RCVD_IN_VALIDITY_CERTIFIED_BLOCKED=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, 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: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Si-Wei Liu writes: > On 5/15/2025 11:40 PM, Markus Armbruster wrote: >> Jason Wang writes: >> >>> On Thu, May 8, 2025 at 2:47=E2=80=AFAM Jonah Palmer wrote: >>>> Current memory operations like pinning may take a lot of time at the >>>> destination. Currently they are done after the source of the migratio= n is >>>> stopped, and before the workload is resumed at the destination. This = is a >>>> period where neigher traffic can flow, nor the VM workload can continue >>>> (downtime). >>>> >>>> We can do better as we know the memory layout of the guest RAM at the >>>> destination from the moment that all devices are initializaed. So >>>> moving that operation allows QEMU to communicate the kernel the maps >>>> while the workload is still running in the source, so Linux can start >>>> mapping them. >>>> >>>> As a small drawback, there is a time in the initialization where QEMU >>>> cannot respond to QMP etc. By some testing, this time is about >>>> 0.2seconds. >>> >>> Adding Markus to see if this is a real problem or not. >> >> I guess the answer is "depends", and to get a more useful one, we need >> more information. >> >> When all you care is time from executing qemu-system-FOO to guest >> finish booting, and the guest takes 10s to boot, then an extra 0.2s >> won't matter much. > > There's no such delay of an extra 0.2s or higher per se, it's just shifti= ng around the page pinning hiccup, no matter it is 0.2s or something else, = from the time of guest booting up to before guest is booted. This saves bac= k guest boot time or start up delay, but in turn the same delay effectively= will be charged to VM launch time. We follow the same model with VFIO, whi= ch would see the same hiccup during launch (at an early stage where no real= mgmt software would care about). > >> When a management application runs qemu-system-FOO several times to >> probe its capabilities via QMP, then even milliseconds can hurt. >> > Not something like that, this page pinning hiccup is one time only that o= ccurs in the very early stage when launching QEMU, i.e. there's no consiste= nt delay every time when QMP is called. The delay in QMP response at that v= ery point depends on how much memory the VM has, but this is just specif to= VM with VFIO or vDPA devices that have to pin memory for DMA. Having said,= there's no extra delay at all if QEMU args has no vDPA device assignment, = on the other hand, there's same delay or QMP hiccup when VFIO is around in = QEMU args. > >> In what scenarios exactly is QMP delayed? > > Having said, this is not a new problem to QEMU in particular, this QMP de= lay is not peculiar, it's existent on VFIO as well. In what scenarios exactly is QMP delayed compared to before the patch? > Thanks, > -Siwei > >> >> You told us an absolute delay you observed. What's the relative delay, >> i.e. what's the delay with and without these patches? Can you answer this question? >> We need QMP to become available earlier in the startup sequence for >> other reasons. Could we bypass the delay that way? Please understand >> that this would likely be quite difficult: we know from experience that >> messing with the startup sequence is prone to introduce subtle >> compatility breaks and even bugs. >> >>> (I remember VFIO has some optimization in the speed of the pinning, >>> could vDPA do the same?) >> >> That's well outside my bailiwick :) >> >> [...] >>