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 44887C61DA4 for ; Thu, 16 Feb 2023 12:22:02 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1pSdGV-00085n-10; Thu, 16 Feb 2023 07:21:31 -0500 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 1pSdGU-00084o-0Q for qemu-devel@nongnu.org; Thu, 16 Feb 2023 07:21:30 -0500 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 1pSdGS-0005yh-8Y for qemu-devel@nongnu.org; Thu, 16 Feb 2023 07:21:29 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1676550087; 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: in-reply-to:in-reply-to:references:references; bh=Q7wgjOkH4EoAmSXdF7LtMEAMjRi09lzI7AGm1QJVQtM=; b=XW1x5NMCEMd/oQE54oBH7/RR3zOItK9DlwZe0hze7fFuf4eJV9aIEB78VFGI5pbtgEST8Q l3RG3Nj6My8/HNO9N8af6uN8IotfGCFrm4y1ATv6MGVdrST2dDrmcJBklh7VKCacZdVh7p GkwmwLYOvQWITJgwKcuZ8LXPEBGa+zg= Received: from mail-wm1-f71.google.com (mail-wm1-f71.google.com [209.85.128.71]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_128_GCM_SHA256) id us-mta-483-NBHjF4lSPDKF8u1WcGu-dA-1; Thu, 16 Feb 2023 07:21:25 -0500 X-MC-Unique: NBHjF4lSPDKF8u1WcGu-dA-1 Received: by mail-wm1-f71.google.com with SMTP id j37-20020a05600c1c2500b003deaf780ab6so1039051wms.4 for ; Thu, 16 Feb 2023 04:21:25 -0800 (PST) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=in-reply-to:content-disposition:mime-version:references:message-id :subject:cc:to:from:date:x-gm-message-state:from:to:cc:subject:date :message-id:reply-to; bh=Q7wgjOkH4EoAmSXdF7LtMEAMjRi09lzI7AGm1QJVQtM=; b=xG4+IT02ANsT4mNO1h3LB0SK5yso1nm0lzwpj5R6kxQTQ1qiGzs0o1jpKcMQ6AsPeF 4+P6r/JbIRd+tKL7ikeqOA5x2mY4BRy42bwJwX8qr8CPE/SR0/KZChJOToZyj9MKl9vx PCnZVeOcPzxEvBy0F2BmeEA1fmELX0HY11oclo3AbW8NyNWRUCxuL04v2tIjqu5zBfzA cMos8V3RUPwFTIm7DTid2emDuE3e3X2xwI9/oG2KKYTT+GmWKuxU4ZfZBoRheAvKyyYM nlYLoTFWY1mfvhJZAJfml/fugEkiR/BdEPwcEm3ST5L9zH+UsrJN2udFvl13p5vm3BLf AjyQ== X-Gm-Message-State: AO0yUKUJtWhu08n7nXfNqjRcVVmXn5d2ifWVWFMB3YyQe+u6IoHT+42J 0yS/c5Ic7bQwYHmMFGMgfcNNo2gaLbjMw9ddznHzbDVNwLkhQiLxsNP4r86ljGTNC2j40IT/VzF 9iT5ke7f8pBT+IRI= X-Received: by 2002:a05:600c:18a6:b0:3e2:f80:3df1 with SMTP id x38-20020a05600c18a600b003e20f803df1mr1478187wmp.19.1676550084798; Thu, 16 Feb 2023 04:21:24 -0800 (PST) X-Google-Smtp-Source: AK7set9ppS6xdslt0BqZjGsZVqX3Cp1RN7U+5o9LOGbbfiTmWvaljIOyAJcO7qQMqSBa3l0PeYD3IA== X-Received: by 2002:a05:600c:18a6:b0:3e2:f80:3df1 with SMTP id x38-20020a05600c18a600b003e20f803df1mr1478170wmp.19.1676550084473; Thu, 16 Feb 2023 04:21:24 -0800 (PST) Received: from redhat.com ([2.52.5.34]) by smtp.gmail.com with ESMTPSA id q19-20020a05600c46d300b003dc434b39c7sm11277259wmo.0.2023.02.16.04.21.22 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 16 Feb 2023 04:21:23 -0800 (PST) Date: Thu, 16 Feb 2023 07:21:20 -0500 From: "Michael S. Tsirkin" To: David Hildenbrand Cc: qemu-devel@nongnu.org, Stefan Hajnoczi , "Dr . David Alan Gilbert" , Tiwei Bie Subject: Re: [PATCH v1 1/2] vhost: Defer filtering memory sections until building the vhost memory structure Message-ID: <20230216072002-mutt-send-email-mst@kernel.org> References: <20230216114752.198627-1-david@redhat.com> <20230216114752.198627-2-david@redhat.com> <20230216070037-mutt-send-email-mst@kernel.org> <0fe7b18c-507a-2c11-8440-e9e35294b4ba@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <0fe7b18c-507a-2c11-8440-e9e35294b4ba@redhat.com> Received-SPF: pass client-ip=170.10.129.124; envelope-from=mst@redhat.com; helo=us-smtp-delivery-124.mimecast.com 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, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H2=-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 On Thu, Feb 16, 2023 at 01:10:54PM +0100, David Hildenbrand wrote: > On 16.02.23 13:04, Michael S. Tsirkin wrote: > > On Thu, Feb 16, 2023 at 12:47:51PM +0100, David Hildenbrand wrote: > > > Having multiple devices, some filtering memslots and some not filtering > > > memslots, messes up the "used_memslot" accounting. If we'd have a device > > > the filters out less memory sections after a device that filters out more, > > > we'd be in trouble, because our memslot checks stop working reliably. > > > For example, hotplugging a device that filters out less memslots might end > > > up passing the checks based on max vs. used memslots, but can run out of > > > memslots when getting notified about all memory sections. > > > > > > Further, it will be helpful in memory device context in the near future > > > to know that a RAM memory region section will consume a memslot, and be > > > accounted for in the used vs. free memslots, such that we can implement > > > reservation of memslots for memory devices properly. Whether a device > > > filters this out and would theoretically still have a free memslot is > > > then hidden internally, making overall vhost memslot accounting easier. > > > > > > Let's filter the memslots when creating the vhost memory array, > > > accounting all RAM && !ROM memory regions as "used_memslots" even if > > > vhost_user isn't interested in anonymous RAM regions, because it needs > > > an fd. > > > > > > When a device actually filters out regions (which should happen rarely > > > in practice), we might detect a layout change although only filtered > > > regions changed. We won't bother about optimizing that for now. > > > > That caused trouble in the past when using VGA because it is playing > > with mappings in weird ways. > > I think we have to optimize it, sorry. > > We still filter them out, just later. The issue is sending lots of unnecessary system calls to update the kernel which goes through a slow RCU. > > > Note: we cannot simply filter out the region and count them as > > > "filtered" to add them to used, because filtered regions could get > > > merged and result in a smaller effective number of memslots. Further, > > > we won't touch the hmp/qmp virtio introspection output. > > > > > > Fixes: 988a27754bbb ("vhost: allow backends to filter memory sections") > > > Cc: Tiwei Bie > > > Signed-off-by: David Hildenbrand > > > > I didn't review this yet but maybe you can answer: > > will this create more slots for the backend? > > Because some backends are limited in # of slots and breaking them is > > not a good idea. > > It restores the handling we had before 988a27754bbb. RAM without an fd > should be rare for vhost-user setups (where we actually filter) I assume? Hmm, I guess so. > -- > Thanks, > > David / dhildenb