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

[Jiri Denemark]

On Wed, Apr 25, 2012 at 17:04:25 -0600, Eric Blake 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.

OK, this seems reasonable, but I'm concerned about backward
compatibility. What if someone is happily using this now with a non-zero
bandwidth even though it's racy? With this change, such usage will fail.

...
> diff --git a/src/qemu/qemu_monitor.h b/src/qemu/qemu_monitor.h
> index f3cdcdd..dc02964 100644
> --- a/src/qemu/qemu_monitor.h
> +++ b/src/qemu/qemu_monitor.h
> @@ -530,8 +530,7 @@ typedef enum {
>      BLOCK_JOB_ABORT = 0,
>      BLOCK_JOB_INFO = 1,
>      BLOCK_JOB_SPEED = 2,
> -    BLOCK_JOB_SPEED_INTERNAL = 3,
> -    BLOCK_JOB_PULL = 4,
> +    BLOCK_JOB_PULL = 3,
>  } BLOCK_JOB_CMD;
...

Why do we use explicit numbering when this is only used internally?

Jirka