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

[Dr. David Alan Gilbert]

* ChenLiang (address@hidden) wrote:
> On 2014/12/10 18:39, Dr. David Alan Gilbert wrote:
> 
> > * Juan Quintela (address@hidden) wrote:
> >> <address@hidden> wrote:
> >>> From: ChenLiang <address@hidden>
> >>>
> >>> Signed-off-by: ChenLiang <address@hidden>
> >>> Signed-off-by: Gonglei <address@hidden>
> >>> Reviewed-by: Dr. David Alan Gilbert <address@hidden>
> >>> ---
> >>>  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
> 
> >
> Hi
> The content that is discussed above is helpful to understand
> the principle of xbzrle. Do you mind that I add it into xbzrle.txt?

That's fine.

Dave

> 
> Best regards
> ChenLiang
> 
> >>>      /* 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 / address@hidden / Manchester, UK
> > 
> > .
> > 
> 
> 
> 
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK