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Subject: Re: [virtio-comment] [PATCH V2 2/2] virtio: introduce STOP status bit
On Fri, Jul 09, 2021 at 07:23:33PM +0200, Eugenio Perez Martin wrote: > > > If I understand correctly, this is all > > > driven from the driver inside the guest, so for this to work > > > the guest must be running and already have initialised the driver. > > > > > > Yes. > > > > As I see it, the feature can be driven entirely by the VMM as long as > it intercept the relevant configuration space (PCI, MMIO, etc) from > guest's reads and writes, and present it as coherent and transparent > for the guest. Some use cases I can imagine with a physical device (or > vp_vpda device) with VIRTIO_F_STOP: > > 1) The VMM chooses not to pass the feature flag. The guest cannot stop > the device, so any write to this flag is an error/undefined. > 2) The VMM passes the flag to the guest. The guest can stop the device. > 2.1) The VMM stops the device to perform a live migration, and the > guest does not write to STOP in any moment of the LM. It resets the > destination device with the state, and then initializes the device. > 2.2) The guest stops the device and, when STOP(32) is set, the source > VMM migrates the device status. The destination VMM realizes the bit, > so it sets the bit in the destination too after device initialization. > 2.3) The device is not initialized by the guest so it doesn't matter > what bit has the HW, but the VM can be migrated. > > Am I missing something? > > Thanks! It's doable like this. It's all a lot of hoops to jump through though. It's also not easy for devices to implement. Why don't we design the feature in a way that is useable by VMMs and implementable by devices in a simple way? -- MST
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