Re: [Qemu-devel] [PATCH v3 09/12] iov: add iov_get_ptr() to reference vector data

[Paolo Bonzini <pbonzini@redhat.com>]

Il 21/11/2012 19:32, Stefan Hajnoczi ha scritto:
> The iov_get_ptr() data returns a pointer to contiguous data within a
> vector.  This allows the caller to manipulate data inside the vector
> without copying in/out using iov_from_buf()/iov_to_buf() when we know
> that data is contiguous within an iovec element.

This works for you because you have a single byte to write.  It would
not work for the SG_IO inhdr, which would need iov_to_buf().

What about the following alternative API:

void *iov_get_ptr(struct iovec *iov, unsigned int iov_cnt,
                  ssize_t offset, size_t *bytes);

which would place the number of valid bytes (i.e. the length of the
remainder of the iovec entry) in *bytes?

Also, I think that offset == iov_size(iov, iov_cnt) should be
acceptable, and it would be the only case in which *bytes == 0.

Otherwise looks good.

Paolo

> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> ---
>  iov.c | 25 +++++++++++++++++++++++++
>  iov.h | 11 +++++++++++
>  2 files changed, 36 insertions(+)
> 
> diff --git a/iov.c b/iov.c
> index 6eed089..bc78c34 100644
> --- a/iov.c
> +++ b/iov.c
> @@ -395,3 +395,28 @@ size_t iov_discard(struct iovec **iov, unsigned int *iov_cnt, ssize_t bytes)
>      }
>      return total;
>  }
> +
> +void *iov_get_ptr(struct iovec *iov, unsigned int iov_cnt,
> +                  ssize_t offset, size_t bytes)
> +{
> +    if (offset < 0) {
> +        offset += iov_size(iov, iov_cnt);
> +        if (offset < 0) {
> +            return NULL; /* offset before beginning of vector */
> +        }
> +    }
> +
> +    while (iov_cnt > 0) {
> +        if (offset < iov->iov_len) {
> +            if (bytes > iov->iov_len - offset) {
> +                return NULL; /* would span iovec elements */
> +            }
> +            return iov->iov_base + offset;
> +        }
> +
> +        offset -= iov->iov_len;
> +        iov_cnt--;
> +        iov++;
> +    }
> +    return NULL; /* offset beyond end of vector */
> +}
> diff --git a/iov.h b/iov.h
> index d6d1fa6..674dd51 100644
> --- a/iov.h
> +++ b/iov.h
> @@ -108,3 +108,14 @@ unsigned iov_copy(struct iovec *dst_iov, unsigned int dst_iov_cnt,
>   * smaller than requested if the vector is too small.
>   */
>  size_t iov_discard(struct iovec **iov, unsigned int *iov_cnt, ssize_t bytes);
> +
> +/*
> + * Get a pointer into a vector at offset if the given number of bytes is
> + * contiguous and not split across iovec elements.  NULL is returned if
> + * memory would span iovec elements or exceed the length of the vector.
> + *
> + * The offset is ssize_t so that an offset from the end of the vector can
> + * be specified with a negative number.
> + */
> +void *iov_get_ptr(struct iovec *iov, unsigned int iov_cnt, ssize_t offset,
> +                  size_t bytes);
>