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

[Markus Armbruster <armbru@redhat.com>]

"Dr. David Alan Gilbert" <dgilbert@redhat.com> writes:

> * 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?).

Lost, not zeroed.  You get whatever the server on the destination put
into shared memory.

>>                                                            I don't yet
>>     know what happens when shared memory setup completes during
>>     migration.

My best guess: it works.

>> 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?

The device model knows, but it's not telling anybody.

> You could delay starting incoming migration on the destination or starting
> a migration on the source until that setup is complete.

That would require new hooks, I guess.

Completing setup in realize() achieves the same effect without such
hackery.
>
> Dave
>
>> 
>> Other ideas?
>> 
> --
> Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK