Re: [Qemu-devel] [PATCH RFC 1/8] virtio: add subsections to the migration stream

[Greg Kurz]

On Thu, 15 May 2014 12:16:35 +0530
Amit Shah <address@hidden> wrote:
> On (Thu) 15 May 2014 [09:23:51], Michael S. Tsirkin wrote:
> > On Thu, May 15, 2014 at 11:34:25AM +0530, Amit Shah wrote:
> > > On (Wed) 14 May 2014 [17:41:38], Greg Kurz wrote:
> > > > There is a need to add some more fields to VirtIODevice that should be
> > > > migrated (broken status, endianness). The problem is that we do not
> > > > want to break compatibility while adding a new feature... This issue has
> > > > been addressed in the generic VMState code with the use of optional
> > > > subsections. As a *temporary* alternative to port the whole virtio
> > > > migration code to VMState, this patch mimics a similar subsectionning
> > > > ability for virtio.
> 
> BTW Greg, do you plan on working on vmstate for virtio?
> 

Yes.

> > > > Since each virtio device is streamed in its own section, the idea is to
> > > > stream subsections between the end of the device section and the start
> > > > of the next sections. This allows an older QEMU to complain and exit
> > > > when fed with subsections:
> > > > 
> > > > Unknown savevm section type 5
> > > > Error -22 while loading VM state
> > > 
> > > Please make this configurable -- either via configure or device
> > > properties.  That avoids having to break existing configurations that
> > > work without this patch.
> > > 
> > > > All users of virtio_load()/virtio_save() need to be patched because the
> > > > subsections are streamed AFTER the device itself.
> > > 
> > > Since all have the same fixup, I'm wondering if a new section can be
> > > added to the virtio-bus itself, which gets propagated to all devices
> > > upon load in the dest.
> > 
> > This calls for a way for devices to inherit properties from the bus,
> > which doesn't exist ATM.
> > Fine but let's not hold up this patchset because of this.
> 
> No, only suggestion is to add a migration section in the bus, and then
> it's easier to do this in the post-migrate functions for each device
> -- so only one new section gets introduced instead of all devices
> being modified to send a new subsection.
> 

The main problem I see is that virtio sucks: as you see in patch 8, we have
to be careful not to call vring or virtqueue stuff before the device knows
its endianness or it breaks... I need to study how the virtio-bus gets
migrated to ensure the endian section is streamed before the devices.

>               Amit
> 

Thanks.

-- 
Gregory Kurz                                     address@hidden
                                                 address@hidden
Software Engineer @ IBM/Meiosys                  http://www.ibm.com
Tel +33 (0)562 165 496

"Anarchy is about taking complete responsibility for yourself."
        Alan Moore.