Re: [Qemu-devel] [PATCH] migration: not send zero page header in ram bulk stage

[Li, Liang Z]

> On 15/01/2016 10:48, Liang Li wrote:
> > Now that VM's RAM pages are initialized to zero, (VM's RAM is allcated
> > with the mmap() and MAP_ANONYMOUS option, or mmap() without
> MAP_SHARED
> > if hugetlbfs is used.) so there is no need to send the zero page
> > header to destination.
> >
> > For guest just uses a small portions of RAM, this change can avoid
> > allocating all the guest's RAM pages in the destination node after
> > live migration. Another benefit is destination QEMU can save lots of
> > CPU cycles for zero page checking.
> >
> > Signed-off-by: Liang Li <address@hidden>
> 
> This does not work.  Depending on the board, some pages are written by
> QEMU before the guest starts.  If the guest rewrites them with zeroes, this
> change breaks migration.
> 
> Paolo

Hi Paolo,

   Luckily I cc to you.  Could you give an example in which case this patch 
will break migration?
Then I can understand your comments better. Much appreciate!


Liang


> 
> > ---
> >  migration/ram.c | 10 ++++++----
> >  1 file changed, 6 insertions(+), 4 deletions(-)
> >
> > diff --git a/migration/ram.c b/migration/ram.c index 4e606ab..c4821d1
> > 100644
> > --- a/migration/ram.c
> > +++ b/migration/ram.c
> > @@ -705,10 +705,12 @@ static int save_zero_page(QEMUFile *f,
> RAMBlock
> > *block, ram_addr_t offset,
> >
> >      if (is_zero_range(p, TARGET_PAGE_SIZE)) {
> >          acct_info.dup_pages++;
> > -        *bytes_transferred += save_page_header(f, block,
> > -                                               offset | 
> > RAM_SAVE_FLAG_COMPRESS);
> > -        qemu_put_byte(f, 0);
> > -        *bytes_transferred += 1;
> > +        if (!ram_bulk_stage) {
> > +            *bytes_transferred += save_page_header(f, block, offset |
> > +                                                   RAM_SAVE_FLAG_COMPRESS);
> > +            qemu_put_byte(f, 0);
> > +            *bytes_transferred += 1;
> > +        }
> >          pages = 1;
> >      }
> >
> >