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Re: [RFC PATCH 0/1] Removing RAMBlocks during migration

From: Yury Kotov
Subject: Re: [RFC PATCH 0/1] Removing RAMBlocks during migration
Date: Mon, 13 Jan 2020 17:18:54 +0300


07.01.2020, 23:08, "Michael S. Tsirkin" <address@hidden>:
> On Fri, Jan 03, 2020 at 11:44:27AM +0000, Dr. David Alan Gilbert wrote:
>>  > 1) Guest: writes to slot's pci config
>>  > 2) QEMU: pcie_cap_slot_write_config -> pcie_unplug_device
>>  >
>>  > So, it's only guest driven action and qdev_unplug doesn't help here.
>>  Hmm we need to find a way to stop that; lets see if Michael Tsirkin has
>>  any ideas (cc'd) - I'm thinking if we could defer the unplug until the
>>  end of the migration we'd be OK; but it feels racy as to whether the
>>  destination is started with the device that the guest is unplugging.
>>  Dave
> It's true - and same is possible with PCI, guest can remove device
> at will.
> Also, while libvirt learned not to start device del while migration
> is active, it's annoying to have to wait for device del
> to complete before migration can be started.
> I guess we can make device invisible to guest - that concept
> now exists because of failover, and can maybe be reused here.
> Alternatively or additionally - for a while now I wanted to only remove
> the device if guest powered it out and removal was requested. Again it
> might be easier now that we have a concept of an invisible device.

I sent an rfc patch that implements deferred device unplug:
  pcie: Defer hot unplug until migration is complete

Please take a look at it.


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