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 Tue, Jan 05, 2016 at 10:01:04AM +0000, Dr. David Alan Gilbert wrote:
> > * 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
> 
> 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

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