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.
> 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?
> 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.
>
> 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?
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
