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Date:   Mon, 1 Feb 2021 11:23:20 +0100
From:   Stefano Garzarella <sgarzare@redhat.com>
To:     Jason Wang <jasowang@redhat.com>
Cc:     virtualization@lists.linux-foundation.org,
        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
Subject: Re: [PATCH RFC v2 04/10] vringh: implement vringh_kiov_advance()
Message-ID: <20210201102320.rk77l2aus3ku4ezz@steredhat>
References: <20210128144127.113245-1-sgarzare@redhat.com>
 <20210128144127.113245-5-sgarzare@redhat.com>
 <78247eb0-8e6e-f2fa-a693-1b0f14db61dd@redhat.com>
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On Mon, Feb 01, 2021 at 01:43:23PM +0800, Jason Wang wrote:
>
>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.

Yes I agree.
I've tried to do this, but it requires quite a bit of work to change 
vringh, I'll put it on my todo list.

Thanks,
Stefano

>
>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;
>>  }
>