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

From: Didier Pallard
Subject: Re: [Qemu-devel] Linux vhost-user interrupt management fixes
Date: Tue, 26 Jan 2016 10:25:54 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Icedove/31.8.0

On 01/25/2016 10:22 AM, Victor Kaplansky wrote:
On Thu, Dec 03, 2015 at 10:53:16AM +0100, Didier Pallard wrote:

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

- 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.


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(-)


thanks for the tests, victor


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