Re: [PATCH V7 16/24] migration: cpr-transfer mode

[Markus Armbruster]

Steve Sistare <steven.sistare@oracle.com> writes:

> Add the cpr-transfer migration mode, which allows the user to transfer
> a guest to a new QEMU instance on the same host with minimal guest pause
> time, by preserving guest RAM in place, albeit with new virtual addresses
> in new QEMU, and by preserving device file descriptors.  Pages that were
> locked in memory for DMA in old QEMU remain locked in new QEMU, because the
> descriptor of the device that locked them remains open.
>
> cpr-transfer preserves memory and devices descriptors by sending them to
> new QEMU over a unix domain socket using SCM_RIGHTS.  Such 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 "cpr" migration channel.  New QEMU reads the cpr channel
> prior to creating devices or backends.  The user specifies the cpr channel
> in the channel arguments on the outgoing side, and in a second -incoming
> command-line parameter on the incoming side.
>
> The user must start old QEMU with the the '-machine aux-ram-share=on' option,
> which allows anonymous memory to be transferred in place to the new process
> by transferring a memory descriptor for each ram block.  Memory-backend
> objects must have the share=on attribute, but memory-backend-epc is not
> supported.
>
> The user starts new QEMU on the same host as old QEMU, with command-line
> arguments to create the same machine, plus the -incoming option for the
> main migration channel, like normal live migration.  In addition, the user
> adds a second -incoming option with channel type "cpr".  This CPR channel
> must support file descriptor transfer with SCM_RIGHTS, i.e. it must be a
> UNIX domain socket.
>
> To initiate CPR, the user issues a migrate command to old QEMU, adding
> a second migration channel of type "cpr" in the channels argument.
> Old QEMU stops the VM, saves state to the migration channels, and enters
> the postmigrate state.  New QEMU mmap's memory descriptors, and execution
> resumes.
>
> The implementation splits qmp_migrate into start and finish functions.
> Start sends CPR state to new QEMU, which responds by closing the CPR
> channel.  Old QEMU detects the HUP then calls finish, which connects the
> main migration channel.
>
> In summary, the usage is:
>
>   qemu-system-$arch -machine aux-ram-share=on ...
>
>   start new QEMU with "-incoming <main-uri> -incoming <cpr-channel>"
>
>   Issue commands to old QEMU:
>     migrate_set_parameter mode cpr-transfer
>
>     {"execute": "migrate", ...
>         {"channel-type": "main"...}, {"channel-type": "cpr"...} ... }
>
> Signed-off-by: Steve Sistare <steven.sistare@oracle.com>
> Reviewed-by: Peter Xu <peterx@redhat.com>

Acked-by: Markus Armbruster <armbru@redhat.com>