Re: [Qemu-block] [PATCH 03/17] migration: split common postcopy out of ram postcopy

[Vladimir Sementsov-Ogievskiy]

24.01.2017 22:53, Dr. David Alan Gilbert wrote:
> * Vladimir Sementsov-Ogievskiy (address@hidden) wrote:
> > 24.01.2017 12:24, Juan Quintela wrote:
> > > Vladimir Sementsov-Ogievskiy <address@hidden> wrote:
> > > > Split common postcopy staff from ram postcopy staff.
> > > >
> > > > Signed-off-by: Vladimir Sementsov-Ogievskiy <address@hidden>
> > > > ---
> > > >    include/migration/migration.h |  1 +
> > > >    migration/migration.c         | 39 +++++++++++++++++++++++++++------------
> > > >    migration/postcopy-ram.c      |  4 +++-
> > > >    migration/savevm.c            | 31 ++++++++++++++++++++++---------
> > > >    4 files changed, 53 insertions(+), 22 deletions(-)
> > > Hi
> > >
> > > {
> > > >        MigrationState *s;
> > > > @@ -1587,9 +1592,11 @@ static int postcopy_start(MigrationState *ms, bool 
> > > > *old_vm_running)
> > > >         * need to tell the destination to throw any pages it's already received
> > > >         * that are dirty
> > > >         */
> > > > -    if (ram_postcopy_send_discard_bitmap(ms)) {
> > > > -        error_report("postcopy send discard bitmap failed");
> > > > -        goto fail;
> > > > +    if (migrate_postcopy_ram()) {
> > > > +        if (ram_postcopy_send_discard_bitmap(ms)) {
> > > > +            error_report("postcopy send discard bitmap failed");
> > > > +            goto fail;
> > > > +        }
> > > I will have preffered that for the ram commands, to embed the
> > > migrate_postocpy_ram() check inside them, but that is taste, and
> > > everyone has its own O:-)
> > >
> > > > diff --git a/migration/savevm.c b/migration/savevm.c
> > > > index e436cb2..c8a71c8 100644
> > > > --- a/migration/savevm.c
> > > > +++ b/migration/savevm.c
> > > > @@ -73,7 +73,7 @@ static struct mig_cmd_args {
> > > >        [MIG_CMD_INVALID]          = { .len = -1, .name = "INVALID" },
> > > >        [MIG_CMD_OPEN_RETURN_PATH] = { .len =  0, .name = "OPEN_RETURN_PATH" },
> > > >        [MIG_CMD_PING]             = { .len = sizeof(uint32_t), .name = "PING" },
> > > > -    [MIG_CMD_POSTCOPY_ADVISE]  = { .len = 16, .name = "POSTCOPY_ADVISE" },
> > > > +    [MIG_CMD_POSTCOPY_ADVISE]  = { .len = -1, .name = "POSTCOPY_ADVISE" },
> > > >        [MIG_CMD_POSTCOPY_LISTEN]  = { .len =  0, .name = "POSTCOPY_LISTEN" },
> > > >        [MIG_CMD_POSTCOPY_RUN]     = { .len =  0, .name = "POSTCOPY_RUN" },
> > > >        [MIG_CMD_POSTCOPY_RAM_DISCARD] = {
> > > > @@ -827,12 +827,17 @@ int qemu_savevm_send_packaged(QEMUFile *f, const uint8_t 
> > > > *buf, size_t len)
> > > >    /* Send prior to any postcopy transfer */
> > > >    void qemu_savevm_send_postcopy_advise(QEMUFile *f)
> > > >    {
> > > > -    uint64_t tmp[2];
> > > > -    tmp[0] = cpu_to_be64(getpagesize());
> > > > -    tmp[1] = cpu_to_be64(1ul << qemu_target_page_bits());
> > > > +    if (migrate_postcopy_ram()) {
> > > > +        uint64_t tmp[2];
> > > > +        tmp[0] = cpu_to_be64(getpagesize());
> > > > +        tmp[1] = cpu_to_be64(1ul << qemu_target_page_bits());
> > > > -    trace_qemu_savevm_send_postcopy_advise();
> > > > -    qemu_savevm_command_send(f, MIG_CMD_POSTCOPY_ADVISE, 16, (uint8_t *)tmp);
> > > > +        trace_qemu_savevm_send_postcopy_advise();
> > > > +        qemu_savevm_command_send(f, MIG_CMD_POSTCOPY_ADVISE,
> > > > +                                 16, (uint8_t *)tmp);
> > > > +    } else {
> > > > +        qemu_savevm_command_send(f, MIG_CMD_POSTCOPY_ADVISE, 0, NULL);
> > > > +    }
> > > >    }
> > > >    /* Sent prior to starting the destination running in postcopy, discard pages
> > > I haven't yet figured out why you are reusing this command with a
> > > different number of parameters.
> > > For this to pass, I need that Dave comment on this.
> > >
> > > So,
> > >
> > > Reviewed-by: Juan Quintela <address@hidden>
> > >
> > > conditioned that Dave agrees with this.
> > These parameters are unrelated if ram postcopy is disabled. So, I should be
> > better to have a possibility of skipping them, then to send unneeded numbers
> > and write separate code to read them from the stream (rewriting
> > loadvm_postcopy_handle_advise to just read these two numbers and do nothing
> > about ram postcopy for this case).
> I think I'd prefer either a new command or keeping these fields (probably all 0 
> ?)
> my worry is what happens in the case if we add a 3rd postcopy subfeature;
> In your case we have three possibilities:
>
>      a) Postcopy RAM only - 16 bytes
>      b) Postcopy persistent-dirty-bitmap only - 0 bytes
>      c) Both - 16 bytes
>
> Lets say we added postcopy-foo in the future and it wanted to add
> another 16 bytes, what would it send if it was foo+persistent-dirty-bitmap and 
> no RAM?
> We'd end up with 16 bytes sent but you'd have to be very careful
> never to get that confused with case (a) above.
>
> (I don't feel too strongly about it though)

Hmm.. Actually I use migrate_postcopy_ram() to distinct these thing in 
loadvm_postcopy_handle_advise.. And it seem like it is a mistake. Are 
migration capabilities available on target?.. On the other hand 
postcopy-test doesn't fail...

> Dave
>
> > --
> > Best regards,
> > Vladimir
> --
> Dr. David Alan Gilbert / address@hidden / Manchester, UK


--
Best regards,
Vladimir