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=-1.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS autolearn=ham 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 3AB22C43387 for ; Wed, 2 Jan 2019 18:02:42 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 11ABD218D3 for ; Wed, 2 Jan 2019 18:02:41 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727068AbfABSCl (ORCPT ); Wed, 2 Jan 2019 13:02:41 -0500 Received: from mx1.redhat.com ([209.132.183.28]:42038 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727031AbfABSCk (ORCPT ); Wed, 2 Jan 2019 13:02:40 -0500 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.phx2.redhat.com [10.5.11.13]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 3165419D4D1; Wed, 2 Jan 2019 18:02:40 +0000 (UTC) Received: from gondolin (ovpn-116-86.ams2.redhat.com [10.36.116.86]) by smtp.corp.redhat.com (Postfix) with ESMTP id E0290608D9; Wed, 2 Jan 2019 18:02:35 +0000 (UTC) Date: Wed, 2 Jan 2019 19:02:33 +0100 From: Cornelia Huck To: Halil Pasic Cc: "Wang, Wei W" , Christian Borntraeger , "virtio-dev@lists.oasis-open.org" , "linux-kernel@vger.kernel.org" , "virtualization@lists.linux-foundation.org" , "kvm@vger.kernel.org" , "mst@redhat.com" , "pbonzini@redhat.com" , "dgilbert@redhat.com" Subject: Re: [virtio-dev] RE: [PATCH v1 0/2] Virtio: fix some vq allocation issues Message-ID: <20190102190233.69b267fb.cohuck@redhat.com> In-Reply-To: <20190102165919.0cd9365a@oc2783563651> References: <1545963986-11280-1-git-send-email-wei.w.wang@intel.com> <286AC319A985734F985F78AFA26841F73DEEA8E9@shsmsx102.ccr.corp.intel.com> <20181230070600.512bbb8b@oc2783563651> <286AC319A985734F985F78AFA26841F73DEEC8DF@shsmsx102.ccr.corp.intel.com> <20190101004019.7f20aafa@oc2783563651> <20190102105314.0b4e2485.cohuck@redhat.com> <20190102142338.15bfe876.cohuck@redhat.com> <20190102165919.0cd9365a@oc2783563651> Organization: Red Hat GmbH MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.29]); Wed, 02 Jan 2019 18:02:40 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 2 Jan 2019 16:59:19 +0100 Halil Pasic wrote: > On Wed, 2 Jan 2019 14:23:38 +0100 > Cornelia Huck wrote: > > > On Wed, 2 Jan 2019 10:53:14 +0100 > > Cornelia Huck wrote: > > > > > On Tue, 1 Jan 2019 00:40:19 +0100 > > > Halil Pasic wrote: > > > > AFAICT tweaking the balloon code may be simpler than tweaking the > > > > virtio-ccw (transport code). ccw_io_helper() relies on getting > > > > an interrupt when the issued IO is done. If virtio-ccw is buggy, it > > > > needs to be fixed, but I'm not sure it is. > > > > > > I would not call virtio-ccw buggy, but it has some constraints that > > > virtio-pci apparently doesn't have (and which did not show up so far; > > > e.g. virtio-blk schedules a work item on config change, so there's no > > > deadlock there.) > > > > > > One way to get out of that constraint (don't interact with the config > > > space directly in the config changed handler) would be to schedule a > > > work item in virtio-ccw that calls virtio_config_changed() for the > > > device. My understanding is that delaying the notification to a work > > > queue would be fine. > > > > Unfortunately, calling virtio_config_changed() from a work item is not > > enough: That function takes the config_lock, and the virtio-ccw code to > > get the config both needs to allocate some memory and call schedule :/ > > > > The best option really seems to be > > - have virtio-balloon move the handling of the config change onto a > > workqueue or something like that, and > > - document that you cannot read/write the virtio config space from an > > atomic context > > > > Unless someone has a better idea? > > > > I wonder, would making config_lock a mutex suffice? Unless I'm mistaken, you can't take a mutex in an interrupt path. > I've already hinted that a virtio-balloon side fix is probably the > simpler variant. Yes, I think so as well. > I agree, let's fix the regression first, and think about wether to teach > virtio-ccw to allow config manipulation from virtio_config_changed() or > not later. Or whether we can tweak the virtio code instead. But I agree, let's get things working again first.