Re: [Qemu-devel] [PATCH] migration: update docs

[Dr. David Alan Gilbert]

* Balamuruhan S (address@hidden) wrote:
> On Fri, Apr 20, 2018 at 06:57:21PM +0100, Dr. David Alan Gilbert (git) wrote:

> > +- Busses and devices should be able to explicitly specify addresses when
> > +  instantiated, and management tools should use those.  For example,
> > +  when hot adding USB devices it's important to specify the ports
> > +  and addresses, since implicit ordering based on the command line order
> > +  may be different on the destination.  This can result in the
> > +  device state being loaded into the wrong device.
> 
> Would you like to add a note about taking care of migrating drc states incase
> of hot adding devices, that could ensure hotunplug device safely after
> migration ?

That's something Power specific as I understand, but I don't know any
details of it.  What would you say as a general warning ?

> > +When we migrate a device, we save/load the state as a series
> > +of fields.  Some times, due to bugs or new functionality, we need to
> > +change the state to store more/different information.  Changing the 
> > migration
> > +state saved for a device can break migration comppatibility unless
> > +care is taken to use the appropriate techniques.  In general QEMU tries
> > +to maintain forward migration compaitibility (i.e. migrating from
> > +QEMU n->n+1) and there are users who benefit from backwards compatibility
> 
> typo - %s/backwards/backward

Interesting, I'd always said 'backwards' and there's only one
place I said 'backward', but I guess 'backward' is more consistent with
'forward' so I'll remove all the 's's

> > +Firmware
> > +========
> > +
> > +Migration migrates the copies of RAM and ROM, and thus when running
> > +on the destination it includes the firmware from the source. Even after
> > +resetting a VM, the old firmware is used.   Only once QEMU has been 
> > restarted
> 
> typo with 2 spaces

I seem to have 3 there; I use 2 in most places; so made it consistent.

Thanks,

Dave

> Only after QEMU has been restarted the new firmware will be used.
> 
> -- Bala
> 
> > +is the new firmware in use.
> > +
> > +- Changes in firmware size can cause changes in the required RAMBlock size
> > +  to hold the firmware and thus migration can fail.  In practice it's best
> > +  to pad firmware images to convenient powers of 2 with plenty of space
> > +  for growth.
> > +
> > +- Care should be taken with device emulation code so that newer
> > +  emulation code can work with older firmware to allow forward migration.
> > +
> > +- Care should be taken with newer firmware so that backwards migration
> > +  to older systems with older device emulation code will work.
> > +
> > +In some cases it may be best to tie specific firmware versions to specific
> > +versioned machine types to cut down on the combinations that will need
> > +support.  This is also useful when newer versions of firmware outgrow
> > +the padding.
> > +
> > -- 
> > 2.17.0
> > 
> > 
> 
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK