Re: [Qemu-devel] [PATCH 2/5] migration: Fix a potential issue

[Li, Liang Z]

> -----Original Message-----
> From: Dr. David Alan Gilbert [mailto:address@hidden
> Sent: Wednesday, May 04, 2016 4:48 PM
> To: Li, Liang Z
> Cc: address@hidden; address@hidden;
> address@hidden; address@hidden
> Subject: Re: [PATCH 2/5] migration: Fix a potential issue
> 
> * Liang Li (address@hidden) wrote:
> > At the end of live migration and before vm_start() on the destination
> > side, we should make sure all the decompression tasks are finished, if
> > this can not be guaranteed, the VM may get the incorrect memory data,
> > or the updated memory may be overwritten by the decompression thread.
> > Add the code to fix this potential issue.
> >
> > Suggested-by: David Alan Gilbert <address@hidden>
> > Signed-off-by: Liang Li <address@hidden>
> > ---
> >  include/migration/migration.h |  1 +
> >  migration/migration.c         |  2 +-
> >  migration/ram.c               | 20 ++++++++++++++++++++
> >  3 files changed, 22 insertions(+), 1 deletion(-)
> >
> > diff --git a/include/migration/migration.h
> > b/include/migration/migration.h index ac2c12c..1c9051e 100644
> > --- a/include/migration/migration.h
> > +++ b/include/migration/migration.h
> > @@ -223,6 +223,7 @@ void migrate_compress_threads_create(void);
> >  void migrate_compress_threads_join(void);
> >  void migrate_decompress_threads_create(void);
> >  void migrate_decompress_threads_join(void);
> > +void wait_for_decompress_done(void);
> >  uint64_t ram_bytes_remaining(void);
> >  uint64_t ram_bytes_transferred(void);  uint64_t
> > ram_bytes_total(void); diff --git a/migration/migration.c
> > b/migration/migration.c index 991313a..5228c28 100644
> > --- a/migration/migration.c
> > +++ b/migration/migration.c
> > @@ -347,7 +347,7 @@ static void process_incoming_migration_bh(void
> *opaque)
> >      /* If global state section was not received or we are in running
> >         state, we need to obey autostart. Any other state is set with
> >         runstate_set. */
> > -
> > +    wait_for_decompress_done();
> 
> I wonder if that's early enough; the roder here is that we get:
> 
>    ram_load
>    devices load
>    wait_for_decompress_done()
>    start VM
> 
> Loading the devices can access guest RAM though (especially virtio); so I
> think you need:
> 
>    ram_load
>    wait_for_decompress_done()
>    devices load
>    start VM
> 
> I think you could do that by placing the wait_for_decompress_done() at the
> end of ram_load().
> 
> Dave

You are right, will change. Thanks!

Liang
> 
> >      if (!global_state_received() ||
> >          global_state_get_runstate() == RUN_STATE_RUNNING) {
> >          if (autostart) {
> > diff --git a/migration/ram.c b/migration/ram.c index 7ab6ab5..4459b38
> > 100644
> > --- a/migration/ram.c
> > +++ b/migration/ram.c
> > @@ -2220,6 +2220,26 @@ static void *do_data_decompress(void *opaque)
> >      return NULL;
> >  }
> >
> > +void wait_for_decompress_done(void)
> > +{
> > +    int idx, thread_count;
> > +
> > +    if (!migrate_use_compression()) {
> > +        return;
> > +    }
> > +    thread_count = migrate_decompress_threads();
> > +    for (idx = 0; idx < thread_count; idx++) {
> > +        if (!decomp_param[idx].done) {
> > +            qemu_mutex_lock(&decomp_done_lock);
> > +            while (!decomp_param[idx].done) {
> > +                qemu_cond_wait(&decomp_done_cond, &decomp_done_lock);
> > +            }
> > +            qemu_mutex_unlock(&decomp_done_lock);
> > +        }
> > +    }
> > +
> > +}
> > +
> >  void migrate_decompress_threads_create(void)
> >  {
> >      int i, thread_count;
> > --
> > 1.9.1
> >
> --
> Dr. David Alan Gilbert / address@hidden / Manchester, UK