On Sun, May 07, 2023 at 03:54:00PM +0300, Avihai Horon wrote:
>
> On 04/05/2023 18:50, Peter Xu wrote:
> > External email: Use caution opening links or attachments
> >
> >
> > On Thu, May 04, 2023 at 01:18:04PM +0300, Avihai Horon wrote:
> > > On 03/05/2023 18:49, Peter Xu wrote:
> > > > External email: Use caution opening links or attachments
> > > >
> > > >
> > > > On Wed, May 03, 2023 at 06:22:59PM +0300, Avihai Horon wrote:
> > > > > On 03/05/2023 1:49, Peter Xu wrote:
> > > > > > External email: Use caution opening links or attachments
> > > > > >
> > > > > >
> > > > > > On Mon, May 01, 2023 at 05:01:33PM +0300, Avihai Horon wrote:
> > > > > > > Hello everyone,
> > > > > > Hi, Avihai,
> > > > > >
> > > > > > > === Flow of operation ===
> > > > > > >
> > > > > > > To use precopy initial data, the capability must be enabled in the
> > > > > > > source.
> > > > > > >
> > > > > > > As this capability must be supported also in the destination, a
> > > > > > > handshake is performed during migration setup. The purpose of the
> > > > > > > handshake is to notify the destination that precopy initial data
> > > > > > > is used
> > > > > > > and to check if it's supported.
> > > > > > >
> > > > > > > The handshake is done in two levels. First, a general handshake
> > > > > > > is done
> > > > > > > with the destination migration code to notify that precopy
> > > > > > > initial data
> > > > > > > is used. Then, for each migration user in the source that supports
> > > > > > > precopy initial data, a handshake is done with its counterpart in
> > > > > > > the
> > > > > > > destination:
> > > > > > > If both support it, precopy initial data will be used for them.
> > > > > > > If source doesn't support it, precopy initial data will not be
> > > > > > > used for
> > > > > > > them.
> > > > > > > If source supports it and destination doesn't, migration will be
> > > > > > > failed.
> > > > > > >
> > > > > > > Assuming the handshake succeeded, migration starts to send
> > > > > > > precopy data
> > > > > > > and as part of it also the initial precopy data. Initial precopy
> > > > > > > data is
> > > > > > > just like any other precopy data and as such, migration code is
> > > > > > > not
> > > > > > > aware of it. Therefore, it's the responsibility of the migration
> > > > > > > users
> > > > > > > (such as VFIO devices) to notify their counterparts in the
> > > > > > > destination
> > > > > > > that their initial precopy data has been sent (for example, VFIO
> > > > > > > migration does it when its initial bytes reach zero).
> > > > > > >
> > > > > > > In the destination, migration code will query each migration user
> > > > > > > that
> > > > > > > supports precopy initial data and check if its initial data has
> > > > > > > been
> > > > > > > loaded. If initial data has been loaded by all of them, an ACK
> > > > > > > will be
> > > > > > > sent to the source which will now be able to complete migration
> > > > > > > when
> > > > > > > appropriate.
> > > > > > I can understand why this is useful, what I'm not 100% sure is
> > > > > > whether the
> > > > > > complexity is needed. The idea seems to be that src never
> > > > > > switchover
> > > > > > unless it receives a READY notification from dst.
> > > > > >
> > > > > > I'm imaging below simplified and more general workflow, not sure
> > > > > > whether it
> > > > > > could work for you:
> > > > > >
> > > > > > - Introduce a new cap "switchover-ready", it means whether
> > > > > > there'll be a
> > > > > > ready event sent from dst -> src for "being ready for
> > > > > > switchover"
> > > > > >
> > > > > > - When cap set, a new msg MIG_RP_MSG_SWITCHOVER_READY is
> > > > > > defined and
> > > > > > handled on src showing that dest is ready for switchover.
> > > > > > It'll be sent
> > > > > > only if dest is ready for the switchover
> > > > > >
> > > > > > - Introduce a field SaveVMHandlers.explicit_switchover_needed.
> > > > > > For each
> > > > > > special device like vfio that would like to participate in
> > > > > > the decision
> > > > > > making, device can set its explicit_switchover_needed=1.
> > > > > > This field is
> > > > > > ignored if the new cap is not set.
> > > > > >
> > > > > > - Dst qemu: when new cap set, remember how many special
> > > > > > devices are there
> > > > > > requesting explicit switchover (count of SaveVMHandlers that
> > > > > > has the
> > > > > > bit set during load setup) as switch_over_pending=N.
> > > > > >
> > > > > > - Dst qemu: Once a device thinks its fine to switchover
> > > > > > (probably in the
> > > > > > load_state() callback), it calls
> > > > > > migration_notify_switchover_ready().
> > > > > > That decreases switch_over_pending and when it hits zero,
> > > > > > one msg
> > > > > > MIG_RP_MSG_SWITCHOVER_READY will be sent to src.
> > > > > >
> > > > > > Only until READY msg received on src could src switchover the
> > > > > > precopy to
> > > > > > dst.
> > > > > >
> > > > > > Then it only needs 1 more field in SaveVMHandlers rather than 3,
> > > > > > and only 1
> > > > > > more msg (dst->src).
> > > > > >
> > > > > > This is based on the fact that right now we always set caps on both
> > > > > > qemus
> > > > > > so I suppose it already means either both have or don't have the
> > > > > > feature
> > > > > > (even if one has, not setting the cap means disabled on both).
> > > > > >
> > > > > > Would it work for this case and cleaner?
> > > > > Hi Peter, thanks for the response!
> > > > > Your approach is indeed much simpler, however I have a few concerns
> > > > > regarding compatibility.
> > > > >
> > > > > You are saying that caps are always set both in src and dest.
> > > > > But what happens if we set the cap only on one side?
> > > > > Should we care about these scenarios?
> > > > I think it's not needed for now, but I am aware that this is a problem.
> > > > It's just that it is a more generic problem to me rather than very
> > > > special
> > > > in the current feature being proposed. At least there're a few times
> > > > Daniel showed concern on keeping this way and hoped we can have a better
> > > > handshake in general with migration framework.
> > > >
> > > > I'd be perfectly fine if you want to approach this with a handshake
> > > > methodology, but I hope if so we should provide a more generic
> > > > handshake.
> > > > So potentially that can make this new feature rely on the handshake
> > > > work,
> > > > and slower to get into shape. Your call on how to address this, at
> > > > least
> > > > fine by me either way.
> > > I'd really like this feature to get in, and I'm afraid making it dependent
> > > on first implementing a general migration handshake may take a long time,
> > > like you said.
> > > What about keeping current approach but changing it such that the
> > > capability
> > > will have to be set in both src and dest, to make it similar to other
> > > capability usages?
> > > I.e., we will remove the "general" handshake:
> > >
> > > /* Enable precopy initial data generally in the migration */
> > > memset(&buf, 0, sizeof(buf));
> > > buf.general_enable = 1;
> > > qemu_savevm_command_send(f, MIG_CMD_INITIAL_DATA_ENABLE, sizeof(buf),
> > > (uint8_t *)&buf);
> > >
> > > but keep the per-device handshake, which is not a handshake for migration
> > > capabilities, but a part of the protocol when the capability is set, like
> > > in
> > > multifd, postcopy, etc.
> > > This way we can advance with this feature while making the general
> > > migration
> > > handshake an independent effort.
> > > Will that work for you?
> > Yes it's fine by me.
> >
> > > BTW, with your suggestion to add a notification mechanism to notify when
> > > initial data is loaded in dest, I think we can drop these two
> > > SaveVMHandlers
> > > handlers:
> > > /*
> > > * Checks if precopy initial data is active. If it's inactive,
> > > * initial_data_loaded check is skipped.
> > > */
> > > bool (*is_initial_data_active)(void *opaque);
> > > /* Checks if precopy initial data has been loaded in dest */
> > > bool (*initial_data_loaded)(void *opaque);
> > >
> > > > In my imagination a generic handshake should happen at the very start of
> > > > migration and negociate feature bits between src/dst qemu, so they can
> > > > reach a consensus on what to do next.
> > > >
> > > > > For example, if we set the cap only in src, then src will wait
> > > > > indefinitely
> > > > > for dest to notify that switchover is ready.
> > > > > Would you expect migration to fail instead of just keep running
> > > > > indefinitely?
> > > > > In current approach we only need to enable the cap in the source, so
> > > > > such
> > > > > scenario can't happen.
> > > > >
> > > > > Let's look at some other scenario.
> > > > > Src QEMU supports explicit-switchover for device X but *not* for
> > > > > device Y
> > > > > (i.e., src QEMU is some older version of QEMU that supports
> > > > > explicit-switchover for device X but not for Y).
> > > > > Dest QEMU supports explicit-switchover for device X and device Y.
> > > > > The capability is set in both src and dest.
> > > > > In the destination we will have switchover_pending=2 because both X
> > > > > and Y
> > > > > support explicit-switchover.
> > > > > We do migration, but switchover_pending will never reach 0 because
> > > > > only X
> > > > > supports it in the source, so the migration will run indefinitely.
> > > > > The per-device handshake solves this by making device Y not use
> > > > > explicit-switchover in this case.
> > > > Hmm, right. When I was replying obviously I thought that decision can
> > > > be
> > > > made sololy by the dest qemu, then I assumed it's fine. Because IIUC in
> > > > that case how many devices that supports switchover_pending on src qemu
> > > > doesn't really matter but only dest.
> > > >
> > > > But I re-read the last patch and I do see that there's a new bit that
> > > > will
> > > > change the device protocol of migration:
> > > >
> > > > if (migration->initial_data_active && !migration->precopy_init_size
> > > > &&
> > > > !migration->initial_data_sent) {
> > > > qemu_put_be64(f, VFIO_MIG_FLAG_DEV_INIT_DATA_SENT);
> > > > migration->initial_data_sent = true;
> > > > } else {
> > > > qemu_put_be64(f, VFIO_MIG_FLAG_END_OF_STATE);
> > > > }
> > > >
> > > > With this, I think what you said makes sense because then the src qemu
> > > > matters on deciding whether to send VFIO_MIG_FLAG_DEV_INIT_DATA_SENT, it
> > > > also needs to make sure dst qemu will recognize it.
> > > >
> > > > Do you think this new VFIO_MIG_FLAG_DEV_INIT_DATA_SENT is a must to
> > > > have?
> > > > Can this decision be made on dest qemu only?
> > > >
> > > > To ask in another way, I saw that precopy_init_size is the fundation to
> > > > decide whether to send this flag. Then it's a matter of whether dest
> > > > qemu
> > > > is also aware of precopy_init_size, then it can already tell when it's
> > > > ready to handle the switchover.
> > > The destination is not aware of precopy_init_size, only the source knows
> > > it.
> > > So the source must send VFIO_MIG_FLAG_DEV_INIT_DATA_SENT to notify dest
> > > that
> > > the initial data was sent.
> > Then, can the src qemu notify instead?
> >
> > We can have similar notification mechanism on src qemu and if that can work
> > we can further same the other MIG_RP_MSG. The counter counts how many
I meant s/same/save/ here..
> > special src devices are there and we don't switchover only if all agree.
>
> Do you mean the following:
> We will have the pending counter and notification mechanism in source
> instead of destination.
> The MIG_RP_MSG_INITIAL_DATA_LOADED_ACK message will be sent by each device
> in the destination that loaded its initial data.
> Each such RP_MSG will decrease the pending counter in source.
> When the pending counter in source reaches zero, source can complete
> migration.
>
> Or did I misunderstand you?
I meant the notification can come sololy from the src qemu, so src qemu can
skip the switchover if any of the src relevant device hasn't yet
acknowledged on the switchover.
Then I think we can avoid introducing a MIG_RP msg? I'm not sure whether
I missed something, though. I stated my understanding on the ordering below.
>
> > I know that even if !precopy_init_size on src, it doesn't mean that dest
> > has already digested all the data in the send buffer. However since we'll
> > anyway make sure queued data landed before switch over happens (e.g., when
> > we only have 1 migration channel data are sent in sequential manner), it
> > means when switchover the dst qemu should have these loaded?
> >
> > Thanks,
> >
> > --
> > Peter Xu
> >
>
--
Peter Xu
