From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:40319) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1eJkHk-0003sk-Mx for qemu-devel@nongnu.org; Tue, 28 Nov 2017 13:07:12 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1eJkHj-0000yC-QX for qemu-devel@nongnu.org; Tue, 28 Nov 2017 13:07:08 -0500 Received: from mx1.redhat.com ([209.132.183.28]:39748) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1eJkHj-0000xa-J0 for qemu-devel@nongnu.org; Tue, 28 Nov 2017 13:07:07 -0500 Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.phx2.redhat.com [10.5.11.15]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 8957FC0587F4 for ; Tue, 28 Nov 2017 18:07:06 +0000 (UTC) Date: Tue, 28 Nov 2017 20:06:52 +0200 From: "Michael S. Tsirkin" Message-ID: <20171128195847-mutt-send-email-mst@kernel.org> References: <20171128133123.17372-1-vkaplans@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20171128133123.17372-1-vkaplans@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: qemu-devel@nongnu.org, Maxime Coquelin , Jason Wang On Tue, Nov 28, 2017 at 03:46:44PM +0200, Victor Kaplansky 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. > > Signed-off-by: Victor Kaplansky > Suggested-by: Maxime Coquelin I'm not sure we can do this. Log resizing as a result of memory hotplug might force log base changes. Backends need to use something like rcu to avoid need for locking. Apropos I wonder whether it's a bug that vhost_dev_start calls vhost_set_log_base after starting rings. Same question for the iotlb callback. > --- > docs/interop/vhost-user.txt | 6 ++++++ > 1 file changed, 6 insertions(+) > > diff --git a/docs/interop/vhost-user.txt b/docs/interop/vhost-user.txt > index 954771d0d8..7ab31e57ef 100644 > --- a/docs/interop/vhost-user.txt > +++ b/docs/interop/vhost-user.txt > @@ -257,6 +257,12 @@ Where addr is the guest physical address. > > Use atomic operations, as the log may be concurrently manipulated. > > +Note that master is not expected to issue more than one VHOST_USER_SET_LOG_BASE > +request before the rings are fully stopped by the master. Thus no modifications > +to log_base address are allowed before the rings are restated and the client > +can ignore all subsequent VHOST_USER_SET_LOG_BASE requests after the log_base > +address has been established. > + > Note that when logging modifications to the used ring (when VHOST_VRING_F_LOG > is set for this ring), log_guest_addr should be used to calculate the log > offset: the write to first byte of the used ring is logged at this offset from > -- > 2.14.2