Re: [PATCH 1/6] migration/postcopy: reduce memset when it is zero page and matches_target_page_size

[Dr. David Alan Gilbert]

* Wei Yang (address@hidden) wrote:
> In this case, page_buffer content would not be used.
> 
> Skip this to save some time.
> 
> Signed-off-by: Wei Yang <address@hidden>
> ---
>  migration/ram.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/migration/ram.c b/migration/ram.c
> index 99a98b2da4..7938a643d9 100644
> --- a/migration/ram.c
> +++ b/migration/ram.c
> @@ -4091,7 +4091,13 @@ static int ram_load_postcopy(QEMUFile *f)
>          switch (flags & ~RAM_SAVE_FLAG_CONTINUE) {
>          case RAM_SAVE_FLAG_ZERO:
>              ch = qemu_get_byte(f);
> -            memset(page_buffer, ch, TARGET_PAGE_SIZE);
> +            /*
> +             * Can skip to set page_buffer when
> +             * this is a zero page and (block->page_size == 
> TARGET_PAGE_SIZE).
> +             */

When it's zero we will use place_page_zero which doesn't need
to page_buffer; so yes that's OK; and you're right, the gotcha is with 
mismatched page sizes where one subpage might be zero but not all of
them; so yes it's right that we need that check.

SO yes,


Reviewed-by: Dr. David Alan Gilbert <address@hidden>

> +            if (ch || !matches_target_page_size) {
> +                memset(page_buffer, ch, TARGET_PAGE_SIZE);
> +            }
>              if (ch) {
>                  all_zero = false;
>              }
> -- 
> 2.17.1
> 
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK