On Fri, Aug 16, 2024 at 11:23:01AM -0400, Peter Xu wrote: > On Fri, Aug 16, 2024 at 11:13:36AM -0400, Steven Sistare wrote: > > On 8/15/2024 4:28 PM, Peter Xu wrote: > > > On Sat, Jul 20, 2024 at 04:07:50PM -0400, Steven Sistare wrote: > > > > > > The new user-visible interfaces are: > > > > > > * cpr-transfer (MigMode migration parameter) > > > > > > * cpr-uri (migration parameter) > > > > > > > > > > I wonder whether this parameter can be avoided already, maybe we can > > > > > let > > > > > cpr-transfer depend on unix socket in -incoming, then integrate fd > > > > > sharing > > > > > in the same channel? > > > > > > > > You saw the answer in another thread, but I repeat it here for others > > > > benefit: > > > > > > > > "CPR state cannot be sent over the normal migration channel, because > > > > devices > > > > and backends are created prior to reading the channel, so this mode > > > > sends > > > > CPR state over a second migration channel that is not visible to > > > > the user. > > > > New QEMU reads the second channel prior to creating devices or > > > > backends." > > > > > > Today when looking again, I wonder about the other way round: can we make > > > the new parameter called "-incoming-cpr", working exactly the same as > > > "cpr-uri" qemu cmdline, but then after cpr is loaded it'll be > > > automatically > > > be reused for migration incoming ports? > > > > > > After all, cpr needs to happen already with unix sockets. Having separate > > > cmdline options grants user to make the other one to be non-unix, but that > > > doesn't seem to buy us anything.. then it seems easier to always reuse it, > > > and restrict cpr-transfer to only work with unix sockets for incoming too? > > > > This idea also occurred to me, but I dislike the loss of flexibility for > > the incoming socket type. The exec URI in particular can do anything, and > > we would be eliminating it. > > Ah, I would be guessing that if Juan is still around then exec URI should > already been marked deprecated and prone to removal soon.. while I tend to > agree that exec does introduce some complexity meanwhile iiuc nobody uses > that in production systems. > > What's the exec use case you're picturing? Would that mostly for debugging > purpose, and would that be easily replaceable with another tunnelling like > "ncat" or so?
Conceptually "exec:" is a nice thing, but from a practical POV it introduces difficulties for QEMU. QEMU doesn't know if the exec'd command will provide a unidirectional channel or bidirectional channel, so has to assume the worst - unidirectional. It also can't know if it is safe to run the exec multiple times, or is only valid to run it once - so afgai nhas to assume once only. We could fix those by adding further flags in the migration address to indicate if its bi-directional & multi-channel safe. Technically "exec" is obsolete given "fd", but then that applies to literally all protocols. Implementing them in QEMU is a more user friendly thing. Exec was more compelling when QEMU's other protocols were less mature, lacking TLS for example, but I still find it interesting as a facility. With regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|
