Re: [Qemu-devel] [PATCH 6/7] migraion: optimiztion xbzrle by reducing data copy

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

* Gonglei (Arei) (arei.gonglei@huawei.com) wrote:
> Reducing data copy can reduce cpu overheah.

(Note a few typos in subject/title)

> Signed-off-by: ChenLiang <chenliang88@huawei.com>
> Signed-off-by: Gonglei <arei.gonglei@huawei.com>
> ---
>  arch_init.c | 8 +++-----
>  1 file changed, 3 insertions(+), 5 deletions(-)
> 
> diff --git a/arch_init.c b/arch_init.c
> index 2211e0b..cc88875 100644
> --- a/arch_init.c
> +++ b/arch_init.c
> @@ -344,11 +344,8 @@ 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);
> -
>      /* 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);

Is xbzrle_encode_buffer safe if the main memory is still being changed while
it is run? Even with multiple CPUs changing it? Even on CPUs with looser
memory ordering semantics?

>      if (encoded_len == 0) {
> @@ -367,7 +364,8 @@ 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 */
> -- 
> 1.7.12.4
> 
> 
> Best regards,
> -Gonglei
> 
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK