Re: [Qemu-devel] [PATCH v4 11/11] virtio-blk: add x-data-plane=on|off performance feature

[Michael S. Tsirkin]

On Thu, Nov 29, 2012 at 04:55:48PM +0200, Michael S. Tsirkin wrote:
> On Thu, Nov 29, 2012 at 03:45:55PM +0100, Stefan Hajnoczi wrote:
> > On Thu, Nov 29, 2012 at 03:12:35PM +0200, Michael S. Tsirkin wrote:
> > > On Thu, Nov 22, 2012 at 04:16:52PM +0100, Stefan Hajnoczi wrote:
> > > > The virtio-blk-data-plane feature is easy to integrate into
> > > > hw/virtio-blk.c.  The data plane can be started and stopped similar to
> > > > vhost-net.
> > > > 
> > > > Users can take advantage of the virtio-blk-data-plane feature using the
> > > > new -device virtio-blk-pci,x-data-plane=on property.
> > > > 
> > > > The x-data-plane name was chosen because at this stage the feature is
> > > > experimental and likely to see changes in the future.
> > > > 
> > > > If the VM configuration does not support virtio-blk-data-plane an error
> > > > message is printed.  Although we could fall back to regular virtio-blk,
> > > > I prefer the explicit approach since it prompts the user to fix their
> > > > configuration if they want the performance benefit of
> > > > virtio-blk-data-plane.
> > > 
> > > Not only that, this affects features exposed to guest so it really can't 
> > > be
> > > trasparent.
> > > 
> > > Which reminds me - shouldn't some features be turned off?
> > > For example, VIRTIO_BLK_F_SCSI?
> > 
> > Yes, virtio-blk-data-plane only starts when you give -device
> > virtio-blk-pci,scsi=off,x-data-plane=on.  If you use scsi=on an error
> > message is printed.
> > 
> > > > Limitations:
> > > >  * Only format=raw is supported
> > > >  * Live migration is not supported
> > > 
> > > This is probably fixable long term?
> > 
> > Absolutely.  There are two parts:
> > 
> > 1. Marking written memory dirty so live RAM migration can work.  Missing
> >    today, easy cheat is to switch off virtio-blk-data-plane and silently
> >    switch to regular virtio-blk emulation while memory dirty logging is
> >    enabled.  The more long-term solution is to actually communicate the
> >    dirty information back to the memory API.
> > 
> > 2. Synchronizing virtio-blk-data-plane vring state with virtio-blk so
> >    save/load works.  This should be relatively straightforward.
> > 
> > I don't want to gate this patch series on live migration support but it
> > is on my TODO list for virtio-blk-data-plane after this initial series
> > has been merged.
> > 
> > > >  * Block jobs, hot unplug, and other operations fail with -EBUSY
> > > 
> > > Hmm I don't see code to disable PCU unplug in this patch.
> > > I expected no_hotplug to be set.
> > > Where is it?
> > 
> > It uses the bdrv_in_use() mechanism.
> 
> Hmm but PCI device can still go away if
> guest ejects it. Does this work fine?

Any comment?