Re: [Qemu-devel] [PATCH 1/2] virtio-balloon: export all balloon statistics

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Feb 25, 2016 at 09:44:06AM +0100, Markus Armbruster wrote:
> "Denis V. Lunev" <den@openvz.org> writes:
> 
> > On 02/24/2016 06:43 PM, Eric Blake wrote:
> >> On 02/24/2016 07:31 AM, Michael S. Tsirkin wrote:
> >>> Roman Kagan <rkagan@virtuozzo.com> writes:
> >>>> On Tue, Feb 23, 2016 at 05:49:21PM +0200, Michael S. Tsirkin wrote:
> >>>>> On Tue, Feb 23, 2016 at 06:29:33PM +0300, Denis V. Lunev wrote:
> >>>>> > On 02/23/2016 06:24 PM, Michael S. Tsirkin wrote:
> >>>>> > >On Tue, Feb 23, 2016 at 05:59:44PM +0300, Denis V. Lunev wrote:
> >>>>> > >>From: Igor Redko <redkoi@virtuozzo.com>
> >>>>> > >>
> >>>>> > >>We are making experiments with different autoballooning strategies
> >>>>> > >>based on the guest behavior. Thus we need to experiment with different
> >>>>> > >>guest statistics. For now every counter change requires QEMU recompilation
> >>>>> > >>and dances with Libvirt.
> >>>>> > >>
> >>>>> > >>This patch introduces transport for unrecognized counters in virtio-balloon.
> >>>>> > >>This transport can be used for measuring benefits from using new
> >>>>> > >>balloon counters, before submitting any patches. Current alternative
> >>>>> > >>is 'guest-exec' transport which isn't made for such delicate matters
> >>>>> > >>and can influence test results.
> >>>>> > >>
> >>>>> > >>Originally all counters with tag >= VIRTIO_BALLOON_S_NR were ignored.
> >>>>> > >>Instead of this we keep first (VIRTIO_BALLOON_S_NR + 32) counters from the
> >>>>> > >>queue and pass unrecognized ones with the following names: 'x-stat-XXXX',
> >>>>> > >>where XXXX is a tag number in hex. Defined counters are reported with their
> >>>>> > >>regular names.
> >>>>> > >>
> >>>>> > >>Signed-off-by: Igor Redko <redkoi@virtuozzo.com>
> >>>>> > >>Signed-off-by: Denis V. Lunev <den@openvz.org>
> >>>>> > >>CC: Michael S. Tsirkin <mst@redhat.com>
> >>>>> > >This seems to open the ABI to abuse.
> >>>>> > >Seems like a reasonable way to experiment though.
> >>>>> > >How about adding this within #if 0 statements?
> >>>>> > >You can uncomment them for debugging ...
> >>>>> > I'd prefer to have this enabled.
> 
> Yes, conditional compilation should be used sparingly.  I don't have an
> opinion on whether using it here is appropriate.
> 
> >>>>> > Why do you think that it opens "abuse" way?
> >>>>> 
> >>>>> Because people will use this to hack drivers and management tools
> >>>>> bypassing qemu.
> 
> Easy to avoid: shuffle the N in x-stat-N around from time to time, to
> reinforce the lesson that you must not rely on their presence or
> semantics.  I doubt it'll be necessary beyond the renumbering that
> happens naturally when we add supported counters, or the reshuffling
> that happens when somebody messes with the unsupported counters.
> 
> >>>> I'm curious why you think it's a problem?  Even the existing stats are
> >>>> simply propagated to the management level by qemu with no processing
> >>>> other than assigning text labels.  The proposed naming scheme for
> >>>> unrecognized counters includes "x-" prefix which explicitly marks them
> >>>> as unstable so people using them take their risk.
> >>>>
> >>>> One of the benefits is forward compatibility, so that counters that have
> >>>> graduated into supported ones and have got their own number and name,
> >>>> can be made to work with qemu that doesn't yet recognize them.
> >>> Then management does start relying on the x- prefixed things,
> >>> and once it's used to that it's a slippery slope.
> >> Any management tool that relies on an x- prefix name is broken.
> 
> Or at least assumes the full risk of breaking without notice whenever
> QEMU changes.  Abbreviating that to just "broken" seems fair enough :)
> 
> >>                                                                  We've
> >> explicitly documented that the x- prefix is unstable and liable to go
> >> away with a future release. Any management app that wants to use a
> >> feature beginning with x- should FIRST push hard to get the x- removed
> >> and stabilize the interface (and libvirt, at least, does just that).
> >>
> > this was exactly an original idea. Names started with 'x-' are
> > _officially_ unstable and for debug purpose. That is why I'd
> > prefer if v2 of the patchset will be taken.
> 
> Looks like fair use of x- to me.


Well I already heard:

	One of the benefits is forward compatibility, so that counters that have
	graduated into supported ones and have got their own number and name,
	can be made to work with qemu that doesn't yet recognize them.

in this thread, which seems to mean exactly that people start planning to abuse it
even before it's merged.

-- 
MST