Re: [Qemu-devel] Re: [PATCH v3 2/3] qerror: Add a new MACHINE_STOPPED error message

[Markus Armbruster]

"Daniel P. Berrange" <address@hidden> writes:

> On Fri, Aug 27, 2010 at 07:39:37AM -0500, Anthony Liguori wrote:
>> On 08/27/2010 04:29 AM, Daniel P. Berrange wrote:
>> >On Fri, Aug 27, 2010 at 10:57:10AM +0530, Amit Shah wrote:
>> >   
>> >>This error message denotes some command was not successful in completing
>> >>as the guest was unresponsive.
>> >>
>> >>Use it in the virtio-balloon code when showing older, cached data.
>> >>
>> >>Signed-off-by: Amit Shah<address@hidden>
>> >>---
>> >>  hw/virtio-balloon.c |    1 +
>> >>  qerror.c            |    4 ++++
>> >>  qerror.h            |    3 +++
>> >>  3 files changed, 8 insertions(+), 0 deletions(-)
>> >>
>> >>diff --git a/hw/virtio-balloon.c b/hw/virtio-balloon.c
>> >>index d6c66cf..309c343 100644
>> >>--- a/hw/virtio-balloon.c
>> >>+++ b/hw/virtio-balloon.c
>> >>@@ -140,6 +140,7 @@ static void complete_stats_request(VirtIOBalloon *vb)
>> >>
>> >>  static void show_old_stats(void *opaque)
>> >>  {
>> >>+    qerror_report(QERR_MACHINE_STOPPED);
>> >>      complete_stats_request(opaque);
>> >>  }
>> >>     
>> >
>> >NACK. It has always been allowed&  valid to call query-balloon
>> >to get the current balloon level. We must not throw an error
>> >just because the recently added mem stats can't be refreshed.
>> 
>> I think that's a fair comment but why even bother fixing the command.  
>> Let's introduce a new command that just gets a single piece of 
>> information instead of having a command return lots of information.
>
> The existing query-balloon command that has been around for years &
> is used by all current apps has a significant regression since we added
> the memstats code to it: a guest can now trivially inflict a DOS on the
> mgmt app if it crashes or is malicious. IMHO we need to fix that regression
> for 0.13 so that existing apps don't suffer[1]. Adding a timeout to silently
> skip the stats refresh if the guest doesn't respond, but without raising
> an error seems the best tradeoff we can do here.

I agree.

Adding a roundtrip through the guest to an existing command was a
mistake.

> Beyond fixing that regression, I agree that this command is terminally
> flawed & we need to deprecate it & provide better specified new
> replacement(s). This seems like 0.14 work to me though.

Yup.

> Regards,
> Daniel
>
> [1] I know that they could already suffer if there was a bug in qemu
>     that prevented it responding, even if the guest was not being
>     malicious/crashed.