Re: [Qemu-devel] [PATCH RESEND for 2.3 5/6] migration: optimize xbzrle by reducing data copy

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Juan Quintela (quintela@redhat.com) wrote:
> <arei.gonglei@huawei.com> wrote:
> > From: ChenLiang <chenliang88@huawei.com>
> >
> > Signed-off-by: ChenLiang <chenliang88@huawei.com>
> > Signed-off-by: Gonglei <arei.gonglei@huawei.com>
> > Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> > ---
> >  arch_init.c | 8 +++-----
> >  1 file changed, 3 insertions(+), 5 deletions(-)
> >
> > diff --git a/arch_init.c b/arch_init.c
> > index 846e4c5..0d0ba4a 100644
> > --- a/arch_init.c
> > +++ b/arch_init.c
> > @@ -376,11 +376,8 @@ static int save_xbzrle_page(QEMUFile *f, uint8_t **current_data,
> >  
> >      prev_cached_page = get_cached_data(XBZRLE.cache, current_addr);
> >  
> > -    /* save current buffer into memory */
> > -    memcpy(XBZRLE.current_buf, *current_data, TARGET_PAGE_SIZE);
> > -
> 
> I think this is wrong.
> Remember that now migration is done in parallel with the guest running.
> If the guest modifies the page while we are encoding it, we end with a
> different contents in the cache and in the real page, and that causes
> corruption.
> 
> This way, what we encoded is a "private copy of the page, so we don't
> have that problem".
> 
> Makes sense?

Kind of; see back in March I hit this while testing the 1st version of this
patch:
https://lists.gnu.org/archive/html/qemu-devel/2014-03/msg05631.html

but then we had some patches that fixed it; and the discussion was here:
https://lists.gnu.org/archive/html/qemu-devel/2014-03/msg05677.html
and then I summarized it as:
https://lists.gnu.org/archive/html/qemu-devel/2014-03/msg05768.html

* It's an interesting, if unusual, observation; it means we can send
* completely bogus data at this point because we know it will get
* overwritten later; I think the requirements are:
* 
*   1) That we meet the protocol (which seems to require that the run lengths are
*      not allowed to be 0)
*   2) That we don't get stuck in any loops or go over the end of the page
*      (I think this means we have to be careful of those byte loops within
*      the word-at-a-time cases)
*   3) The page that ends up in our xbzrle cache must match the destination
*      page, since the next cycle of xbzrle will use it as reference.
* 

Dave

> >      /* XBZRLE encoding (if there is no overflow) */
> > -    encoded_len = xbzrle_encode_buffer(prev_cached_page, XBZRLE.current_buf,
> > +    encoded_len = xbzrle_encode_buffer(prev_cached_page, *current_data,
> >                                         TARGET_PAGE_SIZE, XBZRLE.encoded_buf,
> >                                         TARGET_PAGE_SIZE);
> >      if (encoded_len == 0) {
> > @@ -399,7 +396,8 @@ static int save_xbzrle_page(QEMUFile *f, uint8_t **current_data,
> >  
> >      /* we need to update the data in the cache, in order to get the same data */
> >      if (!last_stage) {
> > -        memcpy(prev_cached_page, XBZRLE.current_buf, TARGET_PAGE_SIZE);
> > +        xbzrle_decode_buffer(XBZRLE.encoded_buf, encoded_len, prev_cached_page,
> > +                             TARGET_PAGE_SIZE);
> >      }
> >  
> >      /* Send XBZRLE based compressed page */
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK