Re: [Qemu-devel] [PATCH v5 11/45] Return path: Send responses from destination to source

[David Gibson]

On Tue, Mar 10, 2015 at 02:34:03PM +0000, Dr. David Alan Gilbert wrote:
> * David Gibson (address@hidden) wrote:
> > On Wed, Feb 25, 2015 at 04:51:34PM +0000, Dr. David Alan Gilbert (git) 
> > wrote:
> > > From: "Dr. David Alan Gilbert" <address@hidden>
> > > 
> > > Add migrate_send_rp_message to send a message from destination to source 
> > > along the return path.
> > >   (It uses a mutex to let it be called from multiple threads)
> > > Add migrate_send_rp_shut to send a 'shut' message to indicate
> > >   the destination is finished with the RP.
> > > Add migrate_send_rp_ack to send a 'PONG' message in response to a PING
> > >   Use it in the CMD_PING handler
> > > 
> > > Signed-off-by: Dr. David Alan Gilbert <address@hidden>
> > > ---
> > >  include/migration/migration.h | 17 ++++++++++++++++
> > >  migration/migration.c         | 45 
> > > +++++++++++++++++++++++++++++++++++++++++++
> > >  savevm.c                      |  2 +-
> > >  trace-events                  |  1 +
> > >  4 files changed, 64 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/include/migration/migration.h b/include/migration/migration.h
> > > index c514dd4..6775747 100644
> > > --- a/include/migration/migration.h
> > > +++ b/include/migration/migration.h
> > > @@ -41,6 +41,13 @@ struct MigrationParams {
> > >      bool shared;
> > >  };
> > >  
> > > +/* Commands sent on the return path from destination to source*/
> > > +enum mig_rpcomm_cmd {
> > 
> > "command" doesn't seem like quite the right description for these rp
> > messages.
> 
> Would you prefer 'message' ?

Perhaps "message type" to distinguish from the the blob including both
tag and data.

> > > +    MIG_RP_CMD_INVALID = 0,  /* Must be 0 */
> > > +    MIG_RP_CMD_SHUT,         /* sibling will not send any more RP 
> > > messages */
> > > +    MIG_RP_CMD_PONG,         /* Response to a PING; data (seq: be32 ) */
> > > +};
> > > +
> > >  typedef struct MigrationState MigrationState;
> > >  
> > >  /* State for the incoming migration */
> > > @@ -48,6 +55,7 @@ struct MigrationIncomingState {
> > >      QEMUFile *file;
> > >  
> > >      QEMUFile *return_path;
> > > +    QemuMutex      rp_mutex;    /* We send replies from multiple threads 
> > > */
> > >  };
> > >  
> > >  MigrationIncomingState *migration_incoming_get_current(void);
> > > @@ -169,6 +177,15 @@ int64_t migrate_xbzrle_cache_size(void);
> > >  
> > >  int64_t xbzrle_cache_resize(int64_t new_size);
> > >  
> > > +/* Sending on the return path - generic and then for each message type */
> > > +void migrate_send_rp_message(MigrationIncomingState *mis,
> > > +                             enum mig_rpcomm_cmd cmd,
> > > +                             uint16_t len, uint8_t *data);
> > > +void migrate_send_rp_shut(MigrationIncomingState *mis,
> > > +                          uint32_t value);
> > > +void migrate_send_rp_pong(MigrationIncomingState *mis,
> > > +                          uint32_t value);
> > > +
> > >  void ram_control_before_iterate(QEMUFile *f, uint64_t flags);
> > >  void ram_control_after_iterate(QEMUFile *f, uint64_t flags);
> > >  void ram_control_load_hook(QEMUFile *f, uint64_t flags);
> > > diff --git a/migration/migration.c b/migration/migration.c
> > > index a36ea65..80d234c 100644
> > > --- a/migration/migration.c
> > > +++ b/migration/migration.c
> > > @@ -78,6 +78,7 @@ MigrationIncomingState 
> > > *migration_incoming_state_new(QEMUFile* f)
> > >  {
> > >      mis_current = g_malloc0(sizeof(MigrationIncomingState));
> > >      mis_current->file = f;
> > > +    qemu_mutex_init(&mis_current->rp_mutex);
> > >  
> > >      return mis_current;
> > >  }
> > > @@ -88,6 +89,50 @@ void migration_incoming_state_destroy(void)
> > >      mis_current = NULL;
> > >  }
> > >  
> > > +/*
> > > + * Send a message on the return channel back to the source
> > > + * of the migration.
> > > + */
> > > +void migrate_send_rp_message(MigrationIncomingState *mis,
> > > +                             enum mig_rpcomm_cmd cmd,
> > > +                             uint16_t len, uint8_t *data)
> > 
> > Using (void *) for data would avoid casts in a bunch of the callers.
> 
> Fixed; thanks.
> 
> > > +{
> > > +    trace_migrate_send_rp_message((int)cmd, len);
> > > +    qemu_mutex_lock(&mis->rp_mutex);
> > > +    qemu_put_be16(mis->return_path, (unsigned int)cmd);
> > > +    qemu_put_be16(mis->return_path, len);
> > > +    qemu_put_buffer(mis->return_path, data, len);
> > > +    qemu_fflush(mis->return_path);
> > > +    qemu_mutex_unlock(&mis->rp_mutex);
> > > +}
> > > +
> > > +/*
> > > + * Send a 'SHUT' message on the return channel with the given value
> > > + * to indicate that we've finished with the RP.  None-0 value indicates
> > > + * error.
> > > + */
> > > +void migrate_send_rp_shut(MigrationIncomingState *mis,
> > > +                          uint32_t value)
> > > +{
> > > +    uint32_t buf;
> > > +
> > > +    buf = cpu_to_be32(value);
> > > +    migrate_send_rp_message(mis, MIG_RP_CMD_SHUT, 4, (uint8_t *)&buf);
> > 
> >                                                      ^ sizeof(buf)
> >                                                  would be safer
> 
> Done.
> 
> > > +}
> > > +
> > > +/*
> > > + * Send a 'PONG' message on the return channel with the given value
> > > + * (normally in response to a 'PING')
> > > + */
> > > +void migrate_send_rp_pong(MigrationIncomingState *mis,
> > > +                          uint32_t value)
> > > +{
> > > +    uint32_t buf;
> > > +
> > > +    buf = cpu_to_be32(value);
> > > +    migrate_send_rp_message(mis, MIG_RP_CMD_PONG, 4, (uint8_t *)&buf);
> > 
> > It occurs to me that you could define PONG as returning the whole
> > buffer that PING sends, instead of just 4-bytes.  Might allow for some
> > more testing of variable sized messages.
> 
> Yes; although it would complicate things a lot if I made it fully generic
> because I'd have to worry about allocating a buffer etc and I'm not
> making vast use of the 4 bytes I've already got.

Couldn't migrate_send_rp_pong just take a buf pointer and length, then
you can point that directly at the buffer in the ping message you've
received.

-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson

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