Re: [Qemu-devel] ivshmem migration restrictions and bugs

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Markus Armbruster (armbru@redhat.com) wrote:
> TL;DR: I recommend to stay away from migration when using chardev=...
> 
> ivshmem migration is messed up in several entertaining ways.
> 
> = General lossage =
> 
> G1. Migrating more than one peer doesn't work, but that's a (badly)
>     documented restriction, not a bug (see documentation of property
>     "role" in qemu-doc.texi).  If you migrate more than one, the shared
>     memory can get messed up.
> 
> G2. If peers connect on the destination before migration is complete,
>     the shared memory can get messed up.  This isn't even badly
>     documented.
> 
> Management applications can deal with this in principle.
> 
> = Lossage with MSI-X (msi=on) =
> 
> M1. s->intrstatus and s->intrmask (registers INTRSTATUS and INTRMASK)
>     are not migrated, even though they have guest-visible contents.
>     They reset to zero instead.  Wrong, but unlikely to cause trouble,
>     because the registers are inert in this configuration.
> 
> There's nothing management applications can do about this.
> 
> = Lossage with interrupts (chardev=...) =
> 
> I1. s->vm_id (register IVPOSITION) is not migrated.  It briefly changes
>     to -1, then to whatever ID the server on the destination assigns.
>     To get the same ID back, you must carefully control the order in
>     which devices connect to the server on the destination: if this
>     device was the n-th to connect on the source, it must also be the
>     n-th on the destination.
> 
>     We can hope that the guest reads IVPOSITION rarely or not at all
>     after device driver initialization, so the temporary change to -1
>     will be overlooked most of the time.
> 
> I2. If the shared memory's ramblock arrives at the destination before
>     shared memory setup completes, migration fails.  Shared memory setup
>     completes shortly after the shared memory is received from the
>     server.
> 
> I3. If migration completes before the shared memory setup completes on
>     the source, shared memory contents is lost (zeroed?).  I don't yet
>     know what happens when shared memory setup completes during
>     migration.
> 
> G2 + I1 implies that you can only migrate the peer with ID zero.
> Management applications need make sure the device with role=master
> connects first both on source and destination, which seems feasible.
> 
> There's nothing management applications can do about the temporary
> IVPOSITION change (I1).
> 
> There is no known way for a management application to wait for shared
> memory setup to complete.
> 
> Migration failure due to I2 is recoverable: restart the server on the
> destination, and retry the migration with a bit more time between
> running the destination QEMU and the migrate command.  The server
> restart is necessary to preserve ID zero.
> 
> I'm not aware of a way to guard against or mitigate I3.  Fortunately,
> shared memory setup should almost always win the race.
> 
> = What can we do about it? =
> 
> G1 and G2 are a matter of improving documentation.
> 
> M1 is easy enough to fix, if we care.
> 
> That leaves I1, I2 and I3.  Common root cause: we don't finish setup in
> realize(), we merely arrange for messages from the server to be received
> and processed.  This exposes both guest and migration to an incompletely
> set up device.
> 
> Completing setup right in realize() would be simpler and race-free.
> However, it could also make realize() hang waiting for a hung server.
> Probably okay for -device, but what about hot plug?
> 
> If it's not okay, we could split ivshmem into a frontend and a backend.
> Hot plug could create the backend asynchronously, wait for it to
> complete, then create the frontend / device model.  Command line would
> have to create the backend synchronously, of course.

How can you tell when 'shared memory setup' is complete?
You could delay starting incoming migration on the destination or starting
a migration on the source until that setup is complete.

Dave

> 
> Other ideas?
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK