From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:59793) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1eJjur-0002Id-3o for qemu-devel@nongnu.org; Tue, 28 Nov 2017 12:43:30 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1eJjun-0007Rk-4b for qemu-devel@nongnu.org; Tue, 28 Nov 2017 12:43:29 -0500 Received: from mx1.redhat.com ([209.132.183.28]:37600) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1eJjum-0007Re-SB for qemu-devel@nongnu.org; Tue, 28 Nov 2017 12:43:25 -0500 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.phx2.redhat.com [10.5.11.14]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 036D5D5541 for ; Tue, 28 Nov 2017 17:43:24 +0000 (UTC) Date: Tue, 28 Nov 2017 17:43:16 +0000 From: "Dr. David Alan Gilbert" Message-ID: <20171128174316.GB3256@work-vm> References: <20171128133123.17372-1-vkaplans@redhat.com> <20171128160420.GG2318@work-vm> <20171128181150-mutt-send-email-mst@kernel.org> <1700185825.47104719.1511890732353.JavaMail.zimbra@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1700185825.47104719.1511890732353.JavaMail.zimbra@redhat.com> Subject: Re: [Qemu-devel] [PATCH] vhost-user spec: Clarify policy on setting log_base List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Victor Kaplansky Cc: "Michael S. Tsirkin" , qemu-devel@nongnu.org, Maxime Coquelin * Victor Kaplansky (vkaplans@redhat.com) wrote: > > From: "Michael S. Tsirkin" > > To: "Dr. David Alan Gilbert" > > Cc: "Victor Kaplansky" , qemu-devel@nongnu.org, "Maxime Coquelin" > > Sent: Tuesday, November 28, 2017 6:12:44 PM > > Subject: Re: [Qemu-devel] [PATCH] vhost-user spec: Clarify policy on setting log_base > > > > On Tue, Nov 28, 2017 at 04:04:21PM +0000, Dr. David Alan Gilbert wrote: > > > * Victor Kaplansky (vkaplans@redhat.com) wrote: > > > > From: Victor Kaplansky > > > > > > > > If we allow qemu to change logging area after it was already established, > > > > it may require from the backend to acquire a lock on each access to > > > > the log_base, which has a potential quite a big performance hit. > > > > > > > > Thus we would like to clarify in the spec, that qemu is not expected > > > > to resize or remap the logging area, and backend implementations > > > > can safely ignore subsequent requests to log_base modifications. > > > > > > There's quite a bit of code in vhost to cope with changes in mappings > > > of regions; and there's already code in there to handle log size > > > changes: > > > > > > static inline void vhost_dev_log_resize(struct vhost_dev *dev, uint64_t > > > size) > > > { > > > struct vhost_log *log = vhost_log_get(size, > > > vhost_dev_log_is_shared(dev)); > > > uint64_t log_base = (uintptr_t)log->log; > > > int r; > > > > > > /* inform backend of log switching, this must be done before > > > releasing the current log, to ensure no logging is lost */ > > > r = dev->vhost_ops->vhost_set_log_base(dev, log_base, log); > > > > > > so when is a log resize legal? > > > > > > Dave > > > > Log base has nothing to do with log resize. > > > > It supplies the physical address of the ring > > for dirty logging purposes. > > > > Right. Just for complete picture, it also supplies the address for > dirty logging of where descriptors can point, in other words for logging > whole guest's physical memory. It is why a single log_base is enough for > whole device, and there is no need to specify it for every ring separately. > > Having said that, there is an obscure possibility to place the ring in > a memory different from guest's physical memory, but as far as I know there > is no backend that supports this now. > > The introduction of this clarification to the vhost-user spec, either with > *must* or without it, will enable us to fix in DPDK several BZs open for qemu > and related to migration failure of MQ vhost-user. OK thanks for the clarification Michael, Victor! Dave -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK