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 05141C54FB3 for ; Mon, 2 Jun 2025 08:09:00 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1uM0Di-0007cX-Vb; Mon, 02 Jun 2025 04:08:35 -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 1uM0Dh-0007cF-CG for qemu-devel@nongnu.org; Mon, 02 Jun 2025 04:08:33 -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 1uM0Dd-0008CC-Kr for qemu-devel@nongnu.org; Mon, 02 Jun 2025 04:08:32 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1748851707; 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=0bUpC5LIZy7c8/oWUe4OiVU3Gk97eiqh6K84Ta7iy1Y=; b=IDGO8V/upMSE/39rfsSaEzd9XPDwL2uf9WLdA65b98NzhsB+7+bjXFumHZN5QIYET2UpC1 s2GjH60kuetQvCkkxQsmbam6ZMdAtjCqmIftXGe+vkRrLoUwINkVyV6gUEoeZPwWLsYpWZ K82Qr1bcM1e1GboS0ejbyF/I/axhk5Q= Received: from mx-prod-mc-05.mail-002.prod.us-west-2.aws.redhat.com (ec2-54-186-198-63.us-west-2.compute.amazonaws.com [54.186.198.63]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-401-Kv1n7RTkOy2GJhK_0Ikm9A-1; Mon, 02 Jun 2025 04:08:24 -0400 X-MC-Unique: Kv1n7RTkOy2GJhK_0Ikm9A-1 X-Mimecast-MFC-AGG-ID: Kv1n7RTkOy2GJhK_0Ikm9A_1748851702 Received: from mx-prod-int-06.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-06.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.93]) (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-05.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 8992D1956086; Mon, 2 Jun 2025 08:08:22 +0000 (UTC) Received: from blackfin.pond.sub.org (unknown [10.45.242.38]) by mx-prod-int-06.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id D46861800368; Mon, 2 Jun 2025 08:08:21 +0000 (UTC) Received: by blackfin.pond.sub.org (Postfix, from userid 1000) id 6728621E66C3; Mon, 02 Jun 2025 10:08:19 +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: (Si-Wei Liu's message of "Thu, 29 May 2025 00:57:30 -0700") References: <20250507184647.15580-1-jonah.palmer@oracle.com> <87plg9ukgq.fsf@pond.sub.org> <50a648fa-76ab-47bf-9f6e-c07da913cb52@oracle.com> <87frgr7mvk.fsf@pond.sub.org> Date: Mon, 02 Jun 2025 10:08:19 +0200 Message-ID: <87o6v6muq4.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 10.30.177.93 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: -31 X-Spam_score: -3.2 X-Spam_bar: --- X-Spam_report: (-3.2 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.071, 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_RPBL_BLOCKED=0.001, RCVD_IN_VALIDITY_SAFE_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 5/26/2025 2:16 AM, Markus Armbruster wrote: >> 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 migrat= ion is >>>>>> stopped, and before the workload is resumed at the destination. Thi= s is a >>>>>> period where neigher traffic can flow, nor the VM workload can conti= nue >>>>>> (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 shif= ting 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 b= ack guest boot time or start up delay, but in turn the same delay effective= ly will be charged to VM launch time. We follow the same model with VFIO, w= hich would see the same hiccup during launch (at an early stage where no re= al 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= occurs in the very early stage when launching QEMU, i.e. there's no consis= tent delay every time when QMP is called. The delay in QMP response at that= very 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 sai= d, 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 i= n QEMU args. >>> >>>> In what scenarios exactly is QMP delayed? >>> >>> Having said, this is not a new problem to QEMU in particular, this QMP = delay is not peculiar, it's existent on VFIO as well. >> >> In what scenarios exactly is QMP delayed compared to before the patch? > > The page pinning process now runs in a pretty early phase at > qemu_init() e.g. machine_run_board_init(), It runs within qemu_init() qmp_x_exit_preconfig() qemu_init_board() machine_run_board_init() Except when --preconfig is given, it instead runs within QMP command x-exit-preconfig. Correct? > before any QMP command can be serviced, the latter of which typically > would be able to get run from qemu_main_loop() until the AIO gets > chance to be started to get polled and dispatched to bh. We create the QMP monitor within qemu_create_late_backends(), which runs before qmp_x_exit_preconfig(), but commands get processed only in the main loop, which we enter later. Correct? > Technically it's not a real delay for specific QMP command, but rather > an extended span of initialization process may take place before the > very first QMP request, usually qmp_capabilities, will be > serviced. It's natural for mgmt software to expect initialization > delay for the first qmp_capabilities response if it has to immediately > issue one after launching qemu, especially when you have a large guest > with hundred GBs of memory and with passthrough device that has to pin > memory for DMA e.g. VFIO, the delayed effect from the QEMU > initialization process is very visible too. > On the other hand, before > the patch, if memory happens to be in the middle of being pinned, any > ongoing QMP can't be serviced by the QEMU main loop, either. > > I'd also like to highlight that without this patch, the pretty high > delay due to page pinning is even visible to the guest in addition to > just QMP delay, which largely affected guest boot time with vDPA > device already. It is long standing, and every VM user with vDPA > device would like to avoid such high delay for the first boot, which > is not seen with similar device e.g. VFIO passthrough. > >> >>> 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? > > I thought I already got that answered in earlier reply. The relative > delay is subject to the size of memory. Usually mgmt software won't be > able to notice, unless the guest has more than 100GB of THP memory to > pin, for DMA or whatever reason. > > >> >>>> 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 :) > > Please be understood that any possible optimization is out of scope of > this patch series, while there's certainly way around that already and > to be carry out in the future, as Peter alluded to in earlier > discussion thread: > > https://lore.kernel.org/qemu-devel/ZZT7wuq-_IhfN_wR@x1n/ > https://lore.kernel.org/qemu-devel/ZZZUNsOVxxqr-H5S@x1n/ > > Thanks, > -Siwei > >>>> >>>> [...] >>>>