Re: [Qemu-devel] [RFC PATCH 0/3] x86: Add support for guest DMA dirty page tracking

[Dr. David Alan Gilbert]

* Michael S. Tsirkin (address@hidden) wrote:
> On Mon, Jan 04, 2016 at 07:11:25PM -0800, Alexander Duyck wrote:
> > >> The two mechanisms referenced above would likely require coordination 
> > >> with
> > >> QEMU and as such are open to discussion.  I haven't attempted to address
> > >> them as I am not sure there is a consensus as of yet.  My personal
> > >> preference would be to add a vendor-specific configuration block to the
> > >> emulated pci-bridge interfaces created by QEMU that would allow us to
> > >> essentially extend shpc to support guest live migration with pass-through
> > >> devices.
> > >
> > > shpc?
> > 
> > That is kind of what I was thinking.  We basically need some mechanism
> > to allow for the host to ask the device to quiesce.  It has been
> > proposed to possibly even look at something like an ACPI interface
> > since I know ACPI is used by QEMU to manage hot-plug in the standard
> > case.
> > 
> > - Alex
> 
> 
> Start by using hot-unplug for this!
> 
> Really use your patch guest side, and write host side
> to allow starting migration with the device, but
> defer completing it.
> 
> So
> 
> 1.- host tells guest to start tracking memory writes
> 2.- guest acks
> 3.- migration starts
> 4.- most memory is migrated
> 5.- host tells guest to eject device
> 6.- guest acks
> 7.- stop vm and migrate rest of state
> 
> 
> It will already be a win since hot unplug after migration starts and
> most memory has been migrated is better than hot unplug before migration
> starts.
> 
> Then measure downtime and profile. Then we can look at ways
> to quiesce device faster which really means step 5 is replaced
> with "host tells guest to quiesce device and dirty (or just unmap!)
> all memory mapped for write by device".


Doing a hot-unplug is going to upset the guests network stacks view
of the world; that's something we don't want to change.

Dave

> 
> -- 
> MST
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK