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Re: [Qemu-devel] Linux vhost-user interrupt management fixes

From: Victor Kaplansky
Subject: Re: [Qemu-devel] Linux vhost-user interrupt management fixes
Date: Mon, 25 Jan 2016 11:22:31 +0200

On Thu, Dec 03, 2015 at 10:53:16AM +0100, 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

Tested-by: Victor Kaplansky <address@hidden>    

> 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(-)
> -- 
> 2.1.4

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