Re: [dpdk-dev] [PATCH 1/2] vhost: enable any layout feature

[Yuanhan Liu <yuanhan.liu@linux.intel.com>]

On Tue, Sep 27, 2016 at 10:56:40PM +0300, Michael S. Tsirkin wrote:
> On Tue, Sep 27, 2016 at 11:11:58AM +0800, Yuanhan Liu wrote:
> > On Mon, Sep 26, 2016 at 10:24:55PM +0300, Michael S. Tsirkin wrote:
> > > On Mon, Sep 26, 2016 at 11:01:58AM -0700, Stephen Hemminger wrote:
> > > > I assume that if using Version 1 that the bit will be ignored
> > 
> > Yes, but I will just quote what you just said: what if the guest
> > virtio device is a legacy device? I also gave my reasons in another
> > email why I consistently set this flag:
> > 
> >   - we have to return all features we support to the guest.
> >   
> >     We don't know the guest is a modern or legacy device. That means
> >     we should claim we support both: VERSION_1 and ANY_LAYOUT.
> >   
> >     Assume guest is a legacy device and we just set VERSION_1 (the current
> >     case), ANY_LAYOUT will never be negotiated.
> >   
> >   - I'm following the way Linux kernel takes: it also set both features.
> >   
> >   Maybe, we could unset ANY_LAYOUT when VERSION_1 is _negotiated_?
> > 
> > The unset after negotiation I proposed turned out it won't work: the
> > feature is already negotiated; unsetting it only in vhost side doesn't
> > change anything. Besides, it may break the migration as Michael stated
> > below.
> 
> I think the reverse. Teach vhost user that for future machine types
> only VERSION_1 implies ANY_LAYOUT.
> 
> 
> > > Therein lies a problem. If dpdk tweaks flags, updating it
> > > will break guest migration.
> > > 
> > > One way is to require that users specify all flags fully when
> > > creating the virtio net device. 
> > 
> > Like how? By a new command line option? And user has to type
> > all those features?
> 
> Make libvirt do this.  users use management normally. those that don't
> likely don't migrate VMs.

Fair enough.

> 
> > > QEMU could verify that all required
> > > flags are set, and fail init if not.
> > > 
> > > This has other advantages, e.g. it adds ability to
> > > init device without waiting for dpdk to connect.

Will the feature negotiation between DPDK and QEMU still exist
in your proposal?

> > > 
> > > However, enabling each new feature would now require
> > > management work. How about dpdk ships the list
> > > of supported features instead?
> > > Management tools could read them on source and destination
> > > and select features supported on both sides.
> > 
> > That means the management tool would somehow has a dependency on
> > DPDK project, which I have no objection at all. But, is that
> > a good idea?
> 
> It already starts the bridge somehow, does it not?

Indeed. I was firstly thinking about reading the dpdk source file
to determine the DPDK supported feature list, with which the bind
is too tight. I later realized you may ask DPDK to provide a binary
to dump the list, or something like that.

> 
> > BTW, I'm not quite sure I followed your idea. I mean, how it supposed
> > to fix the ANY_LAYOUT issue here? How this flag will be set for
> > legacy device?
> > 
> > 	--yliu
> 
> For ANY_LAYOUT, I think we should just set in in qemu,
> but only for new machine types.

What do you mean by "new machine types"? Virtio device with newer
virtio-spec version?

> This addresses migration
> concerns.

To make sure I followed you, do you mean the migration issue from
an older "dpdk + qemu" combo to a newer "dpdk + qemu" combo (that
more new features might be shipped)?

Besides that, your proposal looks like a big work to accomplish.
Are you okay to make it simple first: set it consistently like
what Linux kernel does? This would at least make the ANY_LAYOUT
actually be enabled for legacy device (which is also the default
one that's widely used so far).

	--yliu

> 
> But there will be more new features in the future and
> it is necessary to think how we will enable them without
> breaking migration.
> 
> -- 
> MST