From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:40886) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1ZyE1L-0004Gk-9V for qemu-devel@nongnu.org; Mon, 16 Nov 2015 02:16:12 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1ZyE1I-0005y1-3Q for qemu-devel@nongnu.org; Mon, 16 Nov 2015 02:16:11 -0500 Received: from mx1.redhat.com ([209.132.183.28]:48977) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1ZyE1H-0005xi-Ua for qemu-devel@nongnu.org; Mon, 16 Nov 2015 02:16:08 -0500 Date: Mon, 16 Nov 2015 09:16:05 +0200 From: "Michael S. Tsirkin" Message-ID: <20151116091242-mutt-send-email-mst@redhat.com> References: <002301d11df5$9d440a10$d7cc1e30$@samsung.com> <20151114211725-mutt-send-email-mst@redhat.com> <007f01d1203c$b55d9230$2018b690$@samsung.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <007f01d1203c$b55d9230$2018b690$@samsung.com> Subject: Re: [Qemu-devel] [PATCH] vhost: Fix aborting if KVM does not support eventfds List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Pavel Fedin Cc: qemu-devel@nongnu.org On Mon, Nov 16, 2015 at 10:02:07AM +0300, Pavel Fedin wrote: > Hello! > > > > If you happen to have a stock kernel of old version, like 3.x, and you > > > attempt to enable vhost by setting vhost=on, qemu aborts with error: > > > > > > kvm_mem_ioeventfd_add: error adding ioeventfd: Function not implemented > > > > > > This patch adds capability check, so that vhost gets disabled instead. A > > > warning is displayed, explaining the reason. > > > > > > This problem can be observed with libvirt, which checks for /dev/vhost-net > > > availability and just inserts "vhost=on" automatically in this case. > > > > > > Signed-off-by: Pavel Fedin > > > > How come you have /dev/vhost-net though? > > Easily. Just enable CONFIG_VHOST_NET for the kernel. I happened to have it because i just copied over .config from another kernel > version, which had ioeventfds working. > CONFIG_VHOST_NET in the kernel depends on CONFIG_VHOST, which in turn depends on CONFIG_EVENTFD. But there's no direct dependency > between it and CONFIG_HAVE_KVM_EVENTFD. So, you can just enable CONFIG_IOEVENTFD in "Configure standard kernel features", and get > ioeventfds by themselves with vhost-net. Which, technically speaking, has no direct dependency on KVM's ability to bind ioeventfd to > memory accesses. Because, theoretically, you could have some other use for it. > > > That was supposed to only be there if you have vhost-net, > > and that never existed on kernels without eventfd. > > I believe distro maintainers (should) have taken care of it and disable it. But, as you can see, in some circumstances you could > have it enabled. IMHO it's better to be more flexible and process this situation correctly. The fix is trivial. > > Kind regards, > Pavel Fedin > Expert Engineer > Samsung Electronics Research center Russia > OK so it's a misconfigured kernel. Fine but I'm not happy with silently using userspace instead. If people ask for vhost they should get it. How about - kicking vhost from userspace or - failing vhost init ? -- MST