Re: [PATCH v4 02/16] libs/guest: Reduce number of I/O vectors in write_batch

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Wed, Jun 03, 2026 at 02:05:49PM +0100, Frediano Ziglio wrote:
> From: Frediano Ziglio <frediano.ziglio@xxxxxxxxxx>
> 
> Small optimization.
> Reduce number of pieces passed to writev.
> 
> Signed-off-by: Frediano Ziglio <frediano.ziglio@xxxxxxxxxx>

Reviewed-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

> --
> Changes since v2:
> - change prefix in subject.
> ---
>  tools/libs/guest/xg_sr_save.c | 35 ++++++++++++++++-------------------
>  1 file changed, 16 insertions(+), 19 deletions(-)
> 
> diff --git a/tools/libs/guest/xg_sr_save.c b/tools/libs/guest/xg_sr_save.c
> index 3b2c5222e4..1700d81905 100644
> --- a/tools/libs/guest/xg_sr_save.c
> +++ b/tools/libs/guest/xg_sr_save.c
> @@ -97,9 +97,12 @@ static int write_batch(struct xc_sr_context *ctx)
>      void *page, *orig_page;
>      uint64_t *rec_pfns = NULL;
>      struct iovec *iov = NULL; int iovcnt = 0;
> -    struct xc_sr_rec_page_data_header hdr = { 0 };
> -    struct xc_sr_record rec = {
> -        .type = REC_TYPE_PAGE_DATA,
> +    struct {
> +        struct xc_sr_rhdr rec;
> +        struct xc_sr_rec_page_data_header page_data;

Is there a possible worry that the compiler (for another
architecture) will introduce non-zero padding between those two structs?

> +    } hdrs = {
> +        { .type = REC_TYPE_PAGE_DATA },
> +        { 0 },

Do you need the explicit initialization to 0 here?  All unspecified
fields in the initialization will already be set to 0.

>      };
>  
>      assert(nr_pfns != 0);
> @@ -115,7 +118,7 @@ static int write_batch(struct xc_sr_context *ctx)
>      /* Pointers to locally allocated pages.  Need freeing. */
>      local_pages = calloc(nr_pfns, sizeof(*local_pages));
>      /* iovec[] for writev(). */
> -    iov = malloc((nr_pfns + 4) * sizeof(*iov));
> +    iov = malloc((nr_pfns + 2) * sizeof(*iov));

It would seem more natural to use calloc() here, but it would also do
a zeroing that we don't care.

Thanks, Roger.