Re: [Qemu-devel] [PATCH-RFC 00/13] vhost-net: preview

[Michael S. Tsirkin]

On Mon, Jan 11, 2010 at 08:09:10PM +0000, Daniel P. Berrange wrote:
> On Mon, Jan 11, 2010 at 07:16:42PM +0200, Michael S. Tsirkin wrote:
> > Here's an untested patchset with vhost support for upstream qemu.  Note
> > that you should not expect performance gains from vhost unless in-kernel
> > irqchip is enabled (which is not in upstream qemu now).  Since adding
> > vhost involves quite a bit of infrastructure, I thought it makes sense
> > to send an RFC already, so that interested parties can review it.  In
> > particular, command line and help text need to be finalized early to so
> > that management can start looking on supporting the feature. This patch
> > has all bits besides migration filled in. Also missing is packet socket
> > backend: another team is now working on this.
> 
> Can you clarify a question about migration for me. Is it possible to
> live migrate a guest configured with tap + bridge on one machine over
> to another where it is launched with vhost + bridge, and vice-versa.

Long term, this will be possible without any tweaks.  Currently vhost
does not support mergeable buffers feature which thus needs to be
disabled on both sides for migration to work.  Work is underway to add
this support.

> In other words does this vhost support have any guest visible impact
> that would cause migraiton compatability problems, or it is purely a
> host side optimization like vnet_hdr was ?
> 
> Regards,
> Daniel

vnet_hdr has guest visible impact.

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