From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:35038) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1a4nEU-0001S9-0K for qemu-devel@nongnu.org; Fri, 04 Dec 2015 05:04:55 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1a4nEQ-00085Z-Pv for qemu-devel@nongnu.org; Fri, 04 Dec 2015 05:04:53 -0500 Received: from mail-wm0-x235.google.com ([2a00:1450:400c:c09::235]:34563) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1a4nEQ-00085N-GP for qemu-devel@nongnu.org; Fri, 04 Dec 2015 05:04:50 -0500 Received: by wmvv187 with SMTP id v187so66819852wmv.1 for ; Fri, 04 Dec 2015 02:04:49 -0800 (PST) Message-ID: <5661652F.9060000@6wind.com> Date: Fri, 04 Dec 2015 11:04:31 +0100 From: Didier Pallard MIME-Version: 1.0 References: <1449136399-4158-1-git-send-email-didier.pallard@6wind.com> In-Reply-To: <1449136399-4158-1-git-send-email-didier.pallard@6wind.com> Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] Linux vhost-user interrupt management fixes List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: "qemu-devel"@nongnu.org, mst@redhat.com, pbonzini@redhat.com Cc: thibaut.collet@6wind.com, jmg@6wind.com On 12/03/2015 10:53 AM, Didier Pallard wrote: > Hi, > > I recently did some stress tests of a vhost-user interface using an UDP > traffic generator. Traffic generator was connected to 2 physical ports > that are in turn connected to 2 virtio ports through a linux bridge, VM > (running linux) doing routing to forward packets between the 2 virtio ports. > When traffic reaches high pps rates of small packets, I faced the 2 following > problems: > > - at some time, my qemu socket becomes full, causing qemu to send incomplete > SET_VRING_CALL messages to vhost-user backend (without proper fd set in > ancillary data). > - after some time, some interrupts are lost, causing the VM to stop > transmitting packets. > > Both problems come from the fact that interrupt masking/unmasking of the VM > is deferred to vhost-user backend through the linux socket. > First problem comes from the fact that socket buffer gets full; it is corrected > in the first patch of the serie. > Second problem is a bit more complex. From what i understand of the code, > when VM wants to mask/unmask interrupts, qemu traps the command and sends a > SET_VRING_CALL to the vhost-user to swap interrupt notifications either to > a dummy descriptor or to an fd that was given to kvm module to route > interruption to the VM. After sending SET_VRING_CALL message through > the socket, VM code continues to run, assuming that the interrupts are now > masked; but due to linux socket, this message may be buffered and not currently > treated by the vhost-user backend, ie interrupts are not really masked/unmasked > as they ought to be. > I think it can be solved in two different ways: > - by waiting for an acknowledgement of vhost-user backend before exiting > interrupt masking function, this ensures that interrupt masking is correctly > taken into account before giving back hand to the VM, but it has a very high > cost in term of cycles. Moreover, unless specifying a new option, it will > break current API, since existing vhost-user implementations do not > expect a return message on a SET_VRING_CALL call. > - second way could be, in case vhost-user is involved, to restore the > initial behaviour of interrupt masking (ie masking/unmasking interrupts > by taking/giving vring_call fd from/to kernel kvm module). > Patches 2 and 3 of the serie propose an implementation of this second option. > > Didier > > Didier Pallard (3): > char: fix vhost-user socket full > virtio-pci: add an option to bypass guest_notifier_mask > vhost-net: force guest_notifier_mask bypass in vhost-user case > > hw/net/vhost_net.c | 19 ++++++++++++++++++- > hw/virtio/vhost.c | 13 +++++++++++++ > hw/virtio/virtio-pci.c | 29 +++++++++++++++++++++++------ > hw/virtio/virtio-pci.h | 6 ++++++ > qemu-char.c | 10 ++++++++++ > 5 files changed, 70 insertions(+), 7 deletions(-) > +maintainers sorry, i forgot them in initial mail didier