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 X-Spam-Level: X-Spam-Status: No, score=-3.5 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id B89DDC07E95 for ; Wed, 7 Jul 2021 19:08:29 +0000 (UTC) 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 mail.kernel.org (Postfix) with ESMTPS id 816CC61CCF for ; Wed, 7 Jul 2021 19:08:29 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 816CC61CCF Authentication-Results: mail.kernel.org; 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]:52706 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1m1CuK-0002Yz-O0 for qemu-devel@archiver.kernel.org; Wed, 07 Jul 2021 15:08:28 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:40508) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1m1Ct8-0000vv-72 for qemu-devel@nongnu.org; Wed, 07 Jul 2021 15:07:14 -0400 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:53208) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1m1Ct5-0004Aw-V0 for qemu-devel@nongnu.org; Wed, 07 Jul 2021 15:07:13 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1625684831; 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=ypNvv84iXcVzZehM7AZfSH9krghSc8iJk5sRzmjo2s8=; b=BThmFCOh9U1RRG70lEPMqgxBvO1vmvyycZvrfDpuevok1XcJodYHl0/G9BodOmjSaDP1OJ jRo4XGfcHrnYryNWxzqT902/eTnQpVtXbULgJBrgG/HJukV9fVxuRamDfbnSdfYAbLPI1s B8Aqbrx+DztSsD0WCXVr/eFYhW9nIe0= Received: from mail-ed1-f71.google.com (mail-ed1-f71.google.com [209.85.208.71]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-327-Bx8hc22DPki7MJx2YVdOvQ-1; Wed, 07 Jul 2021 15:07:07 -0400 X-MC-Unique: Bx8hc22DPki7MJx2YVdOvQ-1 Received: by mail-ed1-f71.google.com with SMTP id p19-20020aa7c4d30000b0290394bdda6d9cso1878322edr.21 for ; Wed, 07 Jul 2021 12:07:07 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to; bh=ypNvv84iXcVzZehM7AZfSH9krghSc8iJk5sRzmjo2s8=; b=B0jSX4BEiauqXwRXq/gtgCNKvwfCnaEq/FmSXnEoEnrop0Gm3+DqETUcOHsuFTSTM+ E2E96MmMkK8QhYHKexXW2W80FH3XO+Mc7C/XxtWfED96MuC7UJSxfNNzpPbLmsb7lV19 Kbcw9BDBxXqz7KuYOry6mve9UcFyHJSkl9J5WzTCYAkRXxmFjWl9zVdtwYV4pPtSKo1M U0OWnixY2FgWHrZtokspP8zHMvyTF28UBTjsbJIY2zS0thjpW7T3nX2KBPrjWx3braFx gGXTB35l2e2kDVPqmVDh+MaDiHyHfWJm+v1nbR4L4zpllhK5yYa/krtxTTTk08kdQGSY cDBg== X-Gm-Message-State: AOAM5302j4r1kqK1ySMXEMfdnjKJOFteLer//SqbyMf0kL0CH0d6+maw cYpsv9PVJIwd2wjh0PqCWhtwWkA855U2irU0DE7savzVqEov0/Fq/yi6F4W7hl3N4e4H9oQLcNK iLHrH1/x9FypE8qU= X-Received: by 2002:a17:907:da7:: with SMTP id go39mr26256793ejc.24.1625684826718; Wed, 07 Jul 2021 12:07:06 -0700 (PDT) X-Google-Smtp-Source: ABdhPJyd0JT6NtqFiXmwI6LLOgqrQbCJeQ3SzxV7lKl3bOg3JcSqmyluzbq/HU2+5JKbNtzVvkNnOA== X-Received: by 2002:a17:907:da7:: with SMTP id go39mr26256761ejc.24.1625684826526; Wed, 07 Jul 2021 12:07:06 -0700 (PDT) Received: from redhat.com ([2.55.150.102]) by smtp.gmail.com with ESMTPSA id d13sm9349601eds.56.2021.07.07.12.07.04 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 07 Jul 2021 12:07:06 -0700 (PDT) Date: Wed, 7 Jul 2021 15:07:02 -0400 From: "Michael S. Tsirkin" To: David Hildenbrand Subject: Re: [PATCH v1 2/2] virtio-balloon: disallow postcopy with VIRTIO_BALLOON_F_FREE_PAGE_HINT Message-ID: <20210707150610-mutt-send-email-mst@kernel.org> References: <20210707140655.30982-1-david@redhat.com> <20210707140655.30982-3-david@redhat.com> <5f5dd7f3-ce09-53d6-db48-1a333119205d@redhat.com> MIME-Version: 1.0 In-Reply-To: <5f5dd7f3-ce09-53d6-db48-1a333119205d@redhat.com> Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=mst@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Received-SPF: pass client-ip=216.205.24.124; envelope-from=mst@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -41 X-Spam_score: -4.2 X-Spam_bar: ---- X-Spam_report: (-4.2 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-1.439, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=unavailable 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: Juan Quintela , qemu-devel@nongnu.org, Alexander Duyck , qemu-stable@nongnu.org, Wei Wang , Peter Xu , Philippe =?iso-8859-1?Q?Mathieu-Daud=E9?= , "Dr. David Alan Gilbert" Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Wed, Jul 07, 2021 at 08:57:29PM +0200, David Hildenbrand wrote: > On 07.07.21 20:02, Peter Xu wrote: > > On Wed, Jul 07, 2021 at 04:06:55PM +0200, David Hildenbrand wrote: > > > As it never worked properly, let's disable it via the postcopy notifier on > > > the destination. Trying to set "migrate_set_capability postcopy-ram on" > > > on the destination now results in "virtio-balloon: 'free-page-hint' does > > > not support postcopy Error: Postcopy is not supported". > > > > Would it be possible to do this in reversed order? Say, dynamically disable > > free-page-hinting if postcopy capability is set when migration starts? Perhaps > > it can also be re-enabled automatically when migration completes? > > I remember that this might be quite racy. We would have to make sure that no > hinting happens before we enable the capability. > > As soon as we messed with the dirty bitmap (during precopy), postcopy is no > longer safe. As noted in the patch, the only runtime alternative is to > disable postcopy as soon as we actually do clear a bit. Alternatively, we > could ignore any hints if the postcopy capability was enabled. > > Whatever we do, we have to make sure that a user cannot trick the system > into an inconsistent state. Like enabling hinting, starting migration, then > enabling the postcopy capability and kicking of postcopy. I did not check if > we allow for that, though. What bothers me with limitations like this is we train users about this lack of orthogonality, it's then very hard to retrain them that a given feature is safe to use. > -- > Thanks, > > David / dhildenb