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From: Yuanhan Liu <yuanhan.liu@linux.intel.com>
To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Stephen Hemminger <stephen@networkplumber.org>,
	Maxime Coquelin <maxime.coquelin@redhat.com>,
	dev@dpdk.org, qemu-devel@nongnu.org
Subject: Re: [dpdk-dev] [PATCH 1/2] vhost: enable any layout feature
Date: Wed, 28 Sep 2016 10:28:48 +0800	[thread overview]
Message-ID: <20160928022848.GE1597@yliu-dev.sh.intel.com> (raw)
In-Reply-To: <20160927224935-mutt-send-email-mst@kernel.org>

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

  reply	other threads:[~2016-09-28  2:28 UTC|newest]

Thread overview: 45+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-09-26  6:40 [dpdk-dev] [PATCH 0/2] enables vhost/virtio " Yuanhan Liu
2016-09-26  6:40 ` [dpdk-dev] [PATCH 1/2] vhost: enable " Yuanhan Liu
2016-09-26 18:01   ` Stephen Hemminger
2016-09-26 19:24     ` Michael S. Tsirkin
2016-09-27  3:11       ` Yuanhan Liu
2016-09-27 19:48         ` Stephen Hemminger
2016-09-27 19:56         ` Michael S. Tsirkin
2016-09-28  2:28           ` Yuanhan Liu [this message]
2016-09-29 15:30             ` [dpdk-dev] [Qemu-devel] " Maxime Coquelin
2016-09-29 17:57               ` Michael S. Tsirkin
2016-09-29 20:05                 ` Maxime Coquelin
2016-09-29 20:21                   ` Michael S. Tsirkin
2016-09-29 21:23                     ` Maxime Coquelin
2016-09-30 12:05                       ` Maxime Coquelin
2016-09-30 19:16                         ` Michael S. Tsirkin
2016-10-10  4:05                           ` Yuanhan Liu
2016-10-10  4:17                             ` Michael S. Tsirkin
2016-10-10  4:22                               ` Yuanhan Liu
2016-10-10  4:25                                 ` Michael S. Tsirkin
2016-10-10 12:40                                 ` Maxime Coquelin
2016-10-10 14:42                                   ` Yuanhan Liu
2016-10-10 14:54                                     ` Maxime Coquelin
2016-10-11  6:04                                       ` Yuanhan Liu
2016-10-11  6:39                                         ` Maxime Coquelin
2016-10-11  6:49                                           ` Yuanhan Liu
2016-10-03 14:20                     ` Maxime Coquelin
2016-10-10  3:37                     ` Yuanhan Liu
2016-10-10  3:46                       ` Michael S. Tsirkin
2016-10-10  3:59                         ` Yuanhan Liu
2016-10-10  4:16                           ` Wang, Zhihong
2016-10-10  4:24                             ` Michael S. Tsirkin
2016-10-10  4:39                             ` Michael S. Tsirkin
2016-10-11  6:57                               ` Yuanhan Liu
2016-10-12  3:21                                 ` Yuanhan Liu
     [not found]                                   ` <F5DF4F0E3AFEF648ADC1C3C33AD4DBF16C2409EB@SHSMSX101.ccr.corp.intel.com>
2016-10-13  2:52                                     ` Yang, Zhiyong
2016-10-10  3:50                   ` Yuanhan Liu
2016-10-09 23:20             ` [dpdk-dev] " Michael S. Tsirkin
2016-10-10  3:03               ` Yuanhan Liu
2016-10-10  3:04                 ` Michael S. Tsirkin
2016-10-10  3:10                   ` Yuanhan Liu
2016-09-26  6:40 ` [dpdk-dev] [PATCH 2/2] net/virtio: " Yuanhan Liu
2016-09-26 18:04   ` Stephen Hemminger
2016-09-29 18:00   ` Michael S. Tsirkin
2016-09-29 18:01     ` Michael S. Tsirkin
2016-11-10 15:18 ` [dpdk-dev] [PATCH 0/2] enables vhost/virtio " Michael S. Tsirkin

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