* Michael S. Tsirkin (mst@xxxxxxxxxx) wrote: > 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. > > Which connections and which device? TCP connections and an ethernet > device? These are on different layers so of course you don't lose them. > Just do not change the IP address. > > Some guests send a signal to applications to close connections > when all links go down. One can work around this > in a variety of ways. So, OK, I was surprised that a simple connection didn't go down when I tested and just removed the network card; I'd thought stuff was more aggressive when there was no route. But as you say, some stuff does close connections when the links go down/away so we do need to work around that; and any new outgoing connections get a 'no route to host'. So I'm still nervous what will break. Dave > > > > 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 / dgilbert@xxxxxxxxxx / Manchester, UK > > -- > > Dr. David Alan Gilbert / dgilbert@xxxxxxxxxx / Manchester, UK -- Dr. David Alan Gilbert / dgilbert@xxxxxxxxxx / Manchester, UK -- To unsubscribe from this list: send the line "unsubscribe linux-pci" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
