On Fri, Jan 23, 2026 at 11:10:54AM -0300, Fabiano Rosas wrote:
> Peter Xu <peterx@redhat.com> writes:
>
> > COLO caches all device states in a buffer channel `fb'. Flush it right
> > before pushing the buffer to make sure everything needed will be flushed,
> > add a comment explaning it.
> >
> > Signed-off-by: Peter Xu <peterx@redhat.com>
> > ---
> > migration/colo.c | 4 ++--
> > 1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/migration/colo.c b/migration/colo.c
> > index f92803dd29..e2307f1907 100644
> > --- a/migration/colo.c
> > +++ b/migration/colo.c
> > @@ -472,8 +472,6 @@ static int colo_do_checkpoint_transaction(MigrationState *s,
> > qemu_savevm_state_complete_precopy_iterable(s->to_dst_file, false);
> > qemu_put_byte(s->to_dst_file, QEMU_VM_EOF);
> >
> > - qemu_fflush(fb);
> > -
> > /*
> > * We need the size of the VMstate data in Secondary side,
> > * With which we can decide how much data should be read.
> > @@ -484,6 +482,8 @@ static int colo_do_checkpoint_transaction(MigrationState *s,
> > goto out;
> > }
> >
> > + /* Flush to make sure everything lands bioc->data */
> > + qemu_fflush(fb);
> > qemu_put_buffer(s->to_dst_file, bioc->data, bioc->usage);
> > ret = qemu_fflush(s->to_dst_file);
>
> Reviewed-by: Fabiano Rosas <farosas@suse.de>
>
> Wait, aren't we wasting a memcpy here? Isn't this the same, but without
> the copy of bioc->data to s->to_dst_file->buf:
>
> qemu_fflush(fb); // ensure bioc->data is populated
> qemu_fflush(s->to_dst_file); // send buffered data from migration stream
> qemu_put_buffer_async(s->to_dst_file, bioc->data, ...); // point s->to_dst_file->iov to bioc->data
> qemu_fflush(s->to_dst_file); // send s->to_dst_file->iov contents
Good point! Same to postcopy..
I quickly measured a small VM in postcopy path, async put can shrink the
time needed (plus the flush) from 91us to 38us. Not a huge difference, but
I don't see why we don't do it either. I'll test more and add those when I
repost.
--
Peter Xu