From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:56267) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1dTRpp-0001RU-Er for qemu-devel@nongnu.org; Fri, 07 Jul 2017 07:54:13 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1dTRpm-0001IH-Cc for qemu-devel@nongnu.org; Fri, 07 Jul 2017 07:54:09 -0400 Received: from mx1.redhat.com ([209.132.183.28]:37386) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1dTRpm-0001Hx-63 for qemu-devel@nongnu.org; Fri, 07 Jul 2017 07:54:06 -0400 Date: Fri, 7 Jul 2017 12:53:53 +0100 From: "Dr. David Alan Gilbert" Message-ID: <20170707115353.GD2451@work-vm> References: <20170628190047.26159-1-dgilbert@redhat.com> <20170628190047.26159-25-dgilbert@redhat.com> <2f4c1067-14a4-d943-0dff-790d705778f1@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <2f4c1067-14a4-d943-0dff-790d705778f1@redhat.com> Subject: Re: [Qemu-devel] [RFC 24/29] vhost+postcopy: Lock around set_mem_table List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Maxime Coquelin Cc: qemu-devel@nongnu.org, a.perevalov@samsung.com, marcandre.lureau@redhat.com, mst@redhat.com, quintela@redhat.com, peterx@redhat.com, lvivier@redhat.com, aarcange@redhat.com * Maxime Coquelin (maxime.coquelin@redhat.com) wrote: > > > On 06/28/2017 09:00 PM, Dr. David Alan Gilbert (git) wrote: > > From: "Dr. David Alan Gilbert" > > > > **HACK - better solution needed ** > > We have the situation where: > > > > qemu bridge > > > > send set_mem_table > > map memory > > a) mark area with UFD > > send reply with map addresses > > b) start using > > c) receive reply > > > > As soon as (a) happens qemu might start seeing faults > > from memory accesses (but doesn't until b); but it can't > > process those faults until (c) when it's received the > > mmap addresses. > > > > Make the fault handler spin until it gets the reply in (c). > > > > At the very least this needs some proper locks, but preferably > > we need to split the message. > > Yes, maybe the slave channel could be used to send the ufds with > a dedicated request? The backend would set the reply-ack flag, so that > it starts accessing the guest memory only when Qemu is ready to handle > faults. Yes, that would make life a lot easier. > Note that the slave channel support has not been implemented in Qemu's > libvhost-user yet, but this is something I can do if we feel the need. Can you tell me a bit about how the slave channel works? Dave > Maxime -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK