[libvirt] [PATCH] blockjob: fix block-stream bandwidth race

[Stefan Hajnoczi]

On Thu, Apr 26, 2012 at 12:04 AM, Eric Blake <eblake at redhat.com> wrote:
> With RHEL 6.2, virDomainBlockPull(dom, dev, bandwidth, 0) has a race
> with non-zero bandwidth: there is a window between the block_stream
> and block_job_set_speed monitor commands where an unlimited amount
> of data was let through, defeating the point of a throttle.
>
> This race was first identified in commit a9d3495e, and libvirt was
> able to reduce the size of the window for that race.  In the meantime,
> the qemu developers decided to fix things properly; per this message:
> https://lists.gnu.org/archive/html/qemu-devel/2012-04/msg03793.html
> the fix will be in qemu 1.1, and changes block-job-set-speed to use
> a different parameter name, as well as adding a new optional parameter
> to block-stream, which eliminates the race altogether.
>
> Since our documentation already mentioned that we can refuse a non-zero
> bandwidth for some hypervisors, I think the best solution is to do
> just that for RHEL 6.2 qemu, so that the race is obvious to the user.
> Meanwhile the code must be fixed to honor actual qemu 1.1 naming.
>
> * src/qemu/qemu_monitor.h (enum BLOCK_JOB_CMD): Drop unused value.
> * src/qemu/qemu_driver.c (qemuDomainBlockJobImpl): Reject
> bandwidth during pull with too-old qemu.
> * src/qemu/qemu_monitor_json.c (qemuMonitorJSONBlockJob): Support
> difference between RHEL 6.2 and qemu 1.1 block pull.
> * src/libvirt.c (virDomainBlockPull, virDomainBlockRebase):
> Document this.
> ---
>  src/libvirt.c                |    6 +++-
>  src/qemu/qemu_driver.c       |    8 ++++--
>  src/qemu/qemu_monitor.h      |    3 +-
>  src/qemu/qemu_monitor_json.c |   53 +++++++++++++++++++++++------------------
>  4 files changed, 40 insertions(+), 30 deletions(-)

Reviewed-by: Stefan Hajnoczi <stefanha at linux.vnet.ibm.com>