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Subject: Re: [PATCH RFC v2 04/10] vringh: implement vringh_kiov_advance()
To:     Stefano Garzarella <sgarzare@redhat.com>,
        virtualization@lists.linux-foundation.org
Cc:     Xie Yongji <xieyongji@bytedance.com>,
        "Michael S. Tsirkin" <mst@redhat.com>,
        Laurent Vivier <lvivier@redhat.com>,
        Stefan Hajnoczi <stefanha@redhat.com>,
        linux-kernel@vger.kernel.org, Max Gurtovoy <mgurtovoy@nvidia.com>,
        kvm@vger.kernel.org
References: <20210128144127.113245-1-sgarzare@redhat.com>
 <20210128144127.113245-5-sgarzare@redhat.com>
From:   Jason Wang <jasowang@redhat.com>
Message-ID: <78247eb0-8e6e-f2fa-a693-1b0f14db61dd@redhat.com>
Date:   Mon, 1 Feb 2021 13:43:23 +0800
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On 2021/1/28 下午10:41, Stefano Garzarella wrote:
> In some cases, it may be useful to provide a way to skip a number
> of bytes in a vringh_kiov.
>
> Let's implement vringh_kiov_advance() for this purpose, reusing the
> code from vringh_iov_xfer().
> We replace that code calling the new vringh_kiov_advance().


Acked-by: Jason Wang <jasowang@redhat.com>

In the long run we need to switch to use iov iterator library instead.

Thanks


>
> Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
> ---
>   include/linux/vringh.h |  2 ++
>   drivers/vhost/vringh.c | 41 +++++++++++++++++++++++++++++------------
>   2 files changed, 31 insertions(+), 12 deletions(-)
>
> diff --git a/include/linux/vringh.h b/include/linux/vringh.h
> index 9c077863c8f6..755211ebd195 100644
> --- a/include/linux/vringh.h
> +++ b/include/linux/vringh.h
> @@ -199,6 +199,8 @@ static inline void vringh_kiov_cleanup(struct vringh_kiov *kiov)
>   	kiov->iov = NULL;
>   }
>   
> +void vringh_kiov_advance(struct vringh_kiov *kiov, size_t len);
> +
>   int vringh_getdesc_kern(struct vringh *vrh,
>   			struct vringh_kiov *riov,
>   			struct vringh_kiov *wiov,
> diff --git a/drivers/vhost/vringh.c b/drivers/vhost/vringh.c
> index bee63d68201a..4d800e4f31ca 100644
> --- a/drivers/vhost/vringh.c
> +++ b/drivers/vhost/vringh.c
> @@ -75,6 +75,34 @@ static inline int __vringh_get_head(const struct vringh *vrh,
>   	return head;
>   }
>   
> +/**
> + * vringh_kiov_advance - skip bytes from vring_kiov
> + * @iov: an iov passed to vringh_getdesc_*() (updated as we consume)
> + * @len: the maximum length to advance
> + */
> +void vringh_kiov_advance(struct vringh_kiov *iov, size_t len)
> +{
> +	while (len && iov->i < iov->used) {
> +		size_t partlen = min(iov->iov[iov->i].iov_len, len);
> +
> +		iov->consumed += partlen;
> +		iov->iov[iov->i].iov_len -= partlen;
> +		iov->iov[iov->i].iov_base += partlen;
> +
> +		if (!iov->iov[iov->i].iov_len) {
> +			/* Fix up old iov element then increment. */
> +			iov->iov[iov->i].iov_len = iov->consumed;
> +			iov->iov[iov->i].iov_base -= iov->consumed;
> +
> +			iov->consumed = 0;
> +			iov->i++;
> +		}
> +
> +		len -= partlen;
> +	}
> +}
> +EXPORT_SYMBOL(vringh_kiov_advance);
> +
>   /* Copy some bytes to/from the iovec.  Returns num copied. */
>   static inline ssize_t vringh_iov_xfer(struct vringh *vrh,
>   				      struct vringh_kiov *iov,
> @@ -95,19 +123,8 @@ static inline ssize_t vringh_iov_xfer(struct vringh *vrh,
>   		done += partlen;
>   		len -= partlen;
>   		ptr += partlen;
> -		iov->consumed += partlen;
> -		iov->iov[iov->i].iov_len -= partlen;
> -		iov->iov[iov->i].iov_base += partlen;
>   
> -		if (!iov->iov[iov->i].iov_len) {
> -			/* Fix up old iov element then increment. */
> -			iov->iov[iov->i].iov_len = iov->consumed;
> -			iov->iov[iov->i].iov_base -= iov->consumed;
> -
> -			
> -			iov->consumed = 0;
> -			iov->i++;
> -		}
> +		vringh_kiov_advance(iov, partlen);
>   	}
>   	return done;
>   }