Re: [virtio-comment] [PATCH V2 2/2] virtio: introduce STOP status bit

[Stefan Hajnoczi <stefanha@redhat.com>]

On Tue, Aug 03, 2021 at 02:33:20PM +0800, Jason Wang wrote:
> å 2021/7/26 äå11:07, Stefan Hajnoczi åé:
> > I guess this is just a summary of what we've already discussed and not
> > new information. I think an implementation today would use DBus VMState
> > to transfer implementation-specific device state (an opaque blob).
> 
> 
> Instead of trying to migrate those opaque stuffs which is kind of tricky, I
> wonder if we can avoid them by recording the mapping in the shared
> filesystem itself.

The problem is that virtiofsd has no way of reopening the exact same
files without Linux file handles. So they need to be transferred to the
destination (or stored on a shared file system as you suggested),
regardless of whether they are part of the VIRTIO spec's device state or
not.

Implementation-specific state can be considered outside the scope of the
VIRTIO spec. In other words, we could exclude it from the VIRTIO-level
device state that save/load operate on. This does not solve the problem,
it just shifts the responsibility to the virtualization stack to migrate
this state.

The Linux file handles or other virtiofsd implementation-specific state
would be migrated separately (e.g. using DBus VMstate) so that by the
time the destination device does a VIRTIO load operation, it has the
necessary implementation-specific state ready.

I prefer to support in-band migration of implementation-specific state
because it's less complex to have a single device state instead of
splitting it.

Is this the direction you were thinking in?

Stefan

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