Re: [PATCH v11 09/13] copy-on-read: skip non-guest reads if no copy needed

[Max Reitz <mreitz@redhat.com>]

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On 12.10.20 19:43, Andrey Shinkevich wrote:
> If the flag BDRV_REQ_PREFETCH was set, pass it further to the
> COR-driver to skip unneeded reading. It can be taken into account for
> the COR-algorithms optimization. That check is being made during the
> block stream job by the moment.
> 
> Signed-off-by: Andrey Shinkevich <andrey.shinkevich@virtuozzo.com>
> ---
>  block/copy-on-read.c | 13 +++++++++----
>  block/io.c           |  3 ++-
>  2 files changed, 11 insertions(+), 5 deletions(-)
> 
> diff --git a/block/copy-on-read.c b/block/copy-on-read.c
> index b136895..278a11a 100644
> --- a/block/copy-on-read.c
> +++ b/block/copy-on-read.c
> @@ -148,10 +148,15 @@ static int coroutine_fn cor_co_preadv_part(BlockDriverState *bs,
>              }
>          }
>  
> -        ret = bdrv_co_preadv_part(bs->file, offset, n, qiov, qiov_offset,
> -                                  local_flags);
> -        if (ret < 0) {
> -            return ret;
> +        if (!!(flags & BDRV_REQ_PREFETCH) &

How about dropping the double negation and using a logical && instead of
the binary &?

> +            !(local_flags & BDRV_REQ_COPY_ON_READ)) {
> +            /* Skip non-guest reads if no copy needed */
> +        } else {

Hm.  I would have just written the negated form

(!(flags & BDRV_REQ_PREFETCH) || (local_flags & BDRV_REQ_COPY_ON_READ))

and put the “skip” comment above that condition.

(Since local_flags is initialized to flags, it can be written as a
single comparison, but that’s a matter of taste and I’m not going to
recommend either over the other:

((local_flags & (BDRV_REQ_PREFETCH | BDRV_REQ_COPY_ON_READ)) !=
BDRV_REQ_PREFETCH)

)

> +            ret = bdrv_co_preadv_part(bs->file, offset, n, qiov, qiov_offset,
> +                                      local_flags);
> +            if (ret < 0) {
> +                return ret;
> +            }
>          }
>  
>          offset += n;
> diff --git a/block/io.c b/block/io.c
> index 11df188..bff1808 100644
> --- a/block/io.c
> +++ b/block/io.c
> @@ -1512,7 +1512,8 @@ static int coroutine_fn bdrv_aligned_preadv(BdrvChild *child,
>  
>      max_bytes = ROUND_UP(MAX(0, total_bytes - offset), align);
>      if (bytes <= max_bytes && bytes <= max_transfer) {
> -        ret = bdrv_driver_preadv(bs, offset, bytes, qiov, qiov_offset, 0);
> +        ret = bdrv_driver_preadv(bs, offset, bytes, qiov, qiov_offset,
> +                                 flags & bs->supported_read_flags);

Ah, OK.  I see.  I expected this to be a separate patch.  I still wonder
why it isn’t.

Max

>          goto out;
>      }
>  
> 



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