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

[Michael S. Tsirkin]

On Tue, Jan 05, 2016 at 10:45:25AM +0000, Dr. David Alan Gilbert wrote:
> * Michael S. Tsirkin (mst@xxxxxxxxxx) wrote:
> > On Tue, Jan 05, 2016 at 10:01:04AM +0000, Dr. David Alan Gilbert wrote:
> > > * Michael S. Tsirkin (mst@xxxxxxxxxx) 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
> > 
> > It might but if you store the IP and restore it quickly
> > after migration e.g. using guest agent, as opposed to DHCP,
> > then it won't.
> 
> I thought if you hot-unplug then it will lose any outstanding connections
> on that device.
> 
> > It allows calming the device down in a generic way,
> > specific drivers can then implement the fast quiesce.
> 
> Except that if it breaks the guest networking it's useless.
> 
> Dave

Is hot unplug useless then?

> > 
> > > > 
> > > > -- 
> > > > MST
> > > --
> > > Dr. David Alan Gilbert / dgilbert@xxxxxxxxxx / Manchester, UK
> --
> Dr. David Alan Gilbert / dgilbert@xxxxxxxxxx / Manchester, UK
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