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 B7E1AC388F9 for ; Fri, 23 Oct 2020 16:37:14 +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 E21E7208C3 for ; Fri, 23 Oct 2020 16:37:13 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="K1/Iys18" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org E21E7208C3 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]:38052 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1kW040-0006d7-Qb for qemu-devel@archiver.kernel.org; Fri, 23 Oct 2020 12:37:12 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:40332) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1kVzPv-0000Oz-Ty for qemu-devel@nongnu.org; Fri, 23 Oct 2020 11:55:47 -0400 Received: from us-smtp-delivery-124.mimecast.com ([63.128.21.124]:59765) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_CBC_SHA1:256) (Exim 4.90_1) (envelope-from ) id 1kVzPt-0005Hl-L0 for qemu-devel@nongnu.org; Fri, 23 Oct 2020 11:55:47 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1603468544; 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=iU3NOfsLIQJOHGM/gs3B5H2Cpk1ngy9rHda9it1TQCo=; b=K1/Iys181H//Jq28iXyYzC1TkTgQYGKxF0hLlZeAn98pDJttZXSpxWuS8gQOaru5yh08Yk wYhk3wXZzLkhAH+cR826fPAdtnFUdApvZ4hXPHM9bOj50Tj6MfNiJF8MeoHL05pzovGgNn ZfaxyVlnsj5v77/j+McivadjcMQjtdo= 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-438-8oZyZG29O6SuBh5uSlmo2Q-1; Fri, 23 Oct 2020 11:55:43 -0400 X-MC-Unique: 8oZyZG29O6SuBh5uSlmo2Q-1 Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.phx2.redhat.com [10.5.11.12]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 4246AA6B07; Fri, 23 Oct 2020 15:55:09 +0000 (UTC) Received: from redhat.com (ovpn-113-117.ams2.redhat.com [10.36.113.117]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 55D3A756C5; Fri, 23 Oct 2020 15:54:44 +0000 (UTC) Date: Fri, 23 Oct 2020 11:54:40 -0400 From: "Michael S. Tsirkin" To: Marcel Apfelbaum Subject: Re: [PATCH] pci: Refuse to hotplug PCI Devices when the Guest OS is not ready Message-ID: <20201023115029-mutt-send-email-mst@kernel.org> References: <20201022080354-mutt-send-email-mst@kernel.org> <20201022235632.7f69ddc9@yekko.fritz.box> <20201022100028-mutt-send-email-mst@kernel.org> <20201022102857-mutt-send-email-mst@kernel.org> <20201022110016-mutt-send-email-mst@kernel.org> <20201023144901.5bd908a1@yekko.fritz.box> MIME-Version: 1.0 In-Reply-To: X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 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=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=63.128.21.124; envelope-from=mst@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/10/23 02:46:25 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, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H5=0.001, RCVD_IN_MSPIKE_WL=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: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: David Gibson , Julia Suvorova , qemu devel list Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Fri, Oct 23, 2020 at 09:47:14AM +0300, Marcel Apfelbaum wrote: > Hi David, > > On Fri, Oct 23, 2020 at 6:49 AM David Gibson wrote: > > On Thu, 22 Oct 2020 11:01:04 -0400 > "Michael S. Tsirkin" wrote: > > > On Thu, Oct 22, 2020 at 05:50:51PM +0300, Marcel Apfelbaum wrote: > >  [...]  > > > > Right. After detecting just failing unconditionally it a bit too > > simplistic IMHO. > > There's also another factor here, which I thought I'd mentioned > already, but looks like I didn't: I think we're still missing some > details in what's going on. > > The premise for this patch is that plugging while the indicator is in > transition state is allowed to fail in any way on the guest side.  I > don't think that's a reasonable interpretation, because it's unworkable > for physical hotplug.  If the indicator starts blinking while you're in > the middle of shoving a card in, you'd be in trouble. > > So, what I'm assuming here is that while "don't plug while blinking" is > the instruction for the operator to obey as best they can, on the guest > side the rule has to be "start blinking, wait a while and by the time > you leave blinking state again, you can be confident any plugs or > unplugs have completed".  Obviously still racy in the strict computer > science sense, but about the best you can do with slow humans in the > mix. > > So, qemu should of course endeavour to follow that rule as though it > was a human operator on a physical machine and not plug when the > indicator is blinking.  *But* the qemu plug will in practice be fast > enough that if we're hitting real problems here, it suggests the guest > is still doing something wrong. > > > I personally think there is a little bit of over-engineering here. > Let's start with the spec: > >     Power Indicator Blinking >     A blinking Power Indicator indicates that the slot is powering up or > powering down and that >     insertion or removal of the adapter is not permitted. > > What exactly is an interpretation here? > As you stated, the races are theoretical, the whole point of the indicator > is to let the operator know he can't plug the device just yet. > > I understand it would be more user friendly if the QEMU would wait internally > for the > blinking to end, but the whole point of the indicator is to let the operator  > (human or machine) > know they can't plug the device at a specific time. > Should QEMU take the responsibility of the operator? Is it even correct? > > Even if we would want such a feature, how is it related to this patch? > The patch simply refuses to start a hotplug operation when it knows it will not > succeed.  >   > Another way that would make sense to me would be  is a new QEMU interface other > than > "add_device", let's say "adding_device_allowed", that would return true if the > hotplug is allowed > at this point of time. (I am aware of the theoretical races)  Rather than adding_device_allowed, something like "query slot" might be helpful for debugging. That would help user figure out e.g. why isn't device visible without any races. > The above will at least mimic the mechanics of the pyhs world.  The operator > looks at the indicator, > the management software checks if adding the device is allowed. > Since it is a corner case I would prefer the device_add to fail rather than > introducing a new interface, > but that's just me. > > Thanks, > Marcel > I think we want QEMU management interface to be reasonably abstract and agnostic if possible. Pushing knowledge of hardware detail to management will just lead to pain IMHO. We supported device_add which practically never fails for years, at this point it's easier to keep supporting it than change all users ... > > -- > David Gibson > Principal Software Engineer, Virtualization, Red Hat >