Re: [Qemu-devel] [PATCH v3 2/2] vhost user: Add RARP injection for legacy guest

[Jason Wang <jasowang@redhat.com>]

On 06/23/2015 01:49 PM, Michael S. Tsirkin wrote:
> On Tue, Jun 23, 2015 at 10:12:17AM +0800, Jason Wang wrote:
>> > 
>> > 
>> > On 06/18/2015 11:16 PM, Thibaut Collet wrote:
>>> > > On Tue, Jun 16, 2015 at 10:05 AM, Jason Wang <jasowang@redhat.com> wrote:
>>>> > >>
>>>> > >> On 06/16/2015 03:24 PM, Thibaut Collet wrote:
>>>>> > >>> If my understanding is correct, on a resume operation, we have the
>>>>> > >>> following callback trace:
>>>>> > >>> 1. virtio_pci_restore function that calls all restore call back of
>>>>> > >>> virtio devices
>>>>> > >>> 2. virtnet_restore that calls try_fill_recv function for each virtual queues
>>>>> > >>> 3. try_fill_recv function kicks the virtual queue (through
>>>>> > >>> virtqueue_kick function)
>>>> > >> Yes, but this happens only after pm resume not migration. Migration is
>>>> > >> totally transparent to guest.
>>>> > >>
>>> > > Hi Jason,
>>> > >
>>> > > After a deeper look in the migration code of QEMU a resume event is
>>> > > always sent when the live migration is finished.
>>> > > On a live migration we have the following callback trace:
>>> > > 1. The VM on the new host is set to the state RUN_STATE_INMIGRATE, the
>>> > > autostart boolean to 1  and calls the qemu_start_incoming_migration
>>> > > function (see function main of vl.c)
>>> > > .....
>>> > > 2. call of process_incoming_migration function in
>>> > > migration/migration.c file whatever the way to do the live migration
>>> > > (tcp:, fd:, unix:, exec: ...)
>>> > > 3. call of process_incoming_migration_co function in migration/migration.c
>>> > > 4. call of vm_start function in vl.c (otherwise the migrated VM stay
>>> > > in the pause state, the autostart boolean is set to 1 by the main
>>> > > function in vl.c)
>>> > > 5. call of vm_start function that sets the VM is the RUN_STATE_RUNNING state.
>>> > > 6. call of qapi_event_send_resume function that ends a resume event to the VM
>> > 
>> > AFAIK, this function sends resume event to qemu monitor not VM.
>> > 
>>> > >
>>> > > So when a live migration is ended:
>>> > > 1. a resume event is sent to the guest
>>> > > 2. On the reception of this resume event the virtual queue are kicked
>>> > > by the guest
>>> > > 3. Backend vhost user catches this kick and can emit a RARP to guest
>>> > > that does not support GUEST_ANNOUNCE
>>> > >
>>> > > This solution, as solution based on detection of DRIVER_OK status
>>> > > suggested by Michael, allows backend to send the RARP to legacy guest
>>> > > without involving QEMU and add ioctl to vhost-user.
>> > 
>> > A question here is did vhost-user code pass status to the backend? If
>> > not, how can userspace backend detect DRIVER_OK?
> Sorry, I must have been unclear.
> vhost core calls VHOST_NET_SET_BACKEND on DRIVER_OK.
> Unfortunately vhost user currently translates it to VHOST_USER_NONE.

Looks like VHOST_NET_SET_BACKEND was only used for tap backend.

> As a work around, I think kicking ioeventfds once you get
> VHOST_NET_SET_BACKEND will work.

Maybe just a eventfd_set() in vhost_net_start(). But is this
"workaround" elegant enough to be documented? Is it better to do this
explicitly with a new feature?