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

* Jason Wang (jasowang@redhat.com) wrote:
> å 2021/8/3 äå6:37, Stefan Hajnoczi åé:
> > 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.
> I believe if we want to support live migration of the passthrough
> filesystem. The filesystem itself must be shared? (like NFS)
> Assuming this is true. Can we store those mapping (e.g fuse inode -> host
> inode) in a known path/file in the passthrough filesystem itself and hide
> that file from the guest?

That's pretty dangerous; it assumes that the filesystem is only used
together with virtiofs; as a *shared* filesystem it's possible that it's
being used directly by normal NFS clients as well.
It's also very racy; trying to make sure those mappings reflect the
*current* meaning of inodes even while they're changing under your feet
is non-trivial.

> The destination can simply open this unkown file and do the lookup the
> mapping and reopen the file if necessary.
> Then we don't need the Linux file handle.
> >   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.
> That may work but I want to get rid of the implementation specific stuffs
> like linux handles completely.

I'm not sure how much implementation specific you can get rid of; but
you should be able to comparmentalise it, and you should be able to make
it so that common things can be shared; i.e. if I have two
implementations of virtiofs, both running on Linux, then it might be
good if we can live migrate between them, and standardise the format.

So, I'd expect the core virtiofs data to be standardised globally, then
I'd expect how Linux implementations work to be standardised.


> > 
> > 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.
> I wonder how to deal with migration compatibility in this case.
> > 
> > Is this the direction you were thinking in?
> Somehow.
> Thanks
> > 
> > Stefan
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK

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