From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:40290) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1YxvmB-0004ku-PA for qemu-devel@nongnu.org; Thu, 28 May 2015 07:15:09 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1YxvYU-0002IK-Lm for qemu-devel@nongnu.org; Thu, 28 May 2015 07:02:37 -0400 Message-ID: <5566F545.9070303@redhat.com> Date: Thu, 28 May 2015 13:00:21 +0200 From: Paolo Bonzini MIME-Version: 1.0 References: <1432190583-10518-1-git-send-email-famz@redhat.com> <1432190583-10518-2-git-send-email-famz@redhat.com> <20150526142210.GI24077@noname.str.redhat.com> <55648220.1030206@redhat.com> <20150527090745.GA4669@noname.str.redhat.com> <55659372.9060804@redhat.com> <20150527101016.GB4669@noname.str.redhat.com> <55659FCC.2030704@redhat.com> <20150528024950.GC22609@dhcp-14-238.nay.redhat.com> <20150528094047.GB3385@noname.redhat.com> <20150528105538.GB8461@dhcp-14-238.nay.redhat.com> In-Reply-To: <20150528105538.GB8461@dhcp-14-238.nay.redhat.com> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [PATCH v6 01/13] block: Add op blocker type "device IO" List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Fam Zheng , Kevin Wolf Cc: qemu-block@nongnu.org, qemu-devel@nongnu.org, jcody@redhat.com, armbru@redhat.com, Max Reitz , Stefan Hajnoczi , amit.shah@redhat.com On 28/05/2015 12:55, Fam Zheng wrote: > > Indeed. blk_pause/resume would handle everything in one central place > > in the block layer instead of spreading the logic across all the block > > layer users. > > Sorry, I'm confused. Do you mean there is a way to implement blk_pause > completely in block layer, without the necessity of various notifier handlers > in device models? How would you do that? Do you have to keep a queue of pending requests in the BlockBackend? Since bdrv_drain_all may never return (e.g. stuck NFS connection with nfs=hard), the guest can force unbounded allocation in the host, which is bad. In addition, the BDS doesn't have a list of BlockBackends attached to it. So you need the BlockBackends to register themselves for pause/resume in some way---for example with a notifier list. Then it's irrelevant whether it's the device model or the BB that attaches itself to the notifier list. You can start with doing it in the device models (those that use ioeventfd), and later it can be moved to the BB. The low-level implementation remains the same. Paolo