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

[Juan Quintela]

<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?

>      /* 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 */