Re: [PATCH v2 1/8] iov: add some functions to manage iovec

[David Gibson <david@gibson.dropbear.id.au>]

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On Wed, Feb 14, 2024 at 09:56:21AM +0100, Laurent Vivier wrote:
> Introduce functions to copy to/from a buffer from/to an iovec array,
> to compute data length in in bytes of an iovec and to copy memory from
> an iovec to another.
> 
> iov_from_buf(), iov_to_buf(), iov_size(), iov_copy().
> 
> Signed-off-by: Laurent Vivier <lvivier@redhat.com>
> ---
> 
> Notes:
>     v2:
>        - reorder added files in alphanetical order in Makefile
>        - update comments, cosmetic cleanup
>        - rename iov_from_buf_full/iov_to_buf_full to
>          iov_fill_from_buf/iov_fill_to_buf
>        - split loops that manage offset and bytes copy.
>        - move iov_from_buf()/iov_to_buf() to iov.c
> 
>  Makefile |   8 +--
>  iov.c    | 212 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
>  iov.h    |  43 +++++++++++
>  3 files changed, 259 insertions(+), 4 deletions(-)
>  create mode 100644 iov.c
>  create mode 100644 iov.h
> 
> diff --git a/Makefile b/Makefile
> index af4fa87e7e13..156398b3844e 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -45,16 +45,16 @@ FLAGS += -DVERSION=\"$(VERSION)\"
>  FLAGS += -DDUAL_STACK_SOCKETS=$(DUAL_STACK_SOCKETS)
>  
>  PASST_SRCS = arch.c arp.c checksum.c conf.c dhcp.c dhcpv6.c flow.c icmp.c \
> -	igmp.c isolation.c lineread.c log.c mld.c ndp.c netlink.c packet.c \
> -	passt.c pasta.c pcap.c pif.c port_fwd.c tap.c tcp.c tcp_splice.c udp.c \
> -	util.c
> +	igmp.c iov.c isolation.c lineread.c log.c mld.c ndp.c netlink.c \
> +	packet.c passt.c pasta.c pcap.c pif.c port_fwd.c tap.c tcp.c \
> +	tcp_splice.c udp.c util.c
>  QRAP_SRCS = qrap.c
>  SRCS = $(PASST_SRCS) $(QRAP_SRCS)
>  
>  MANPAGES = passt.1 pasta.1 qrap.1
>  
>  PASST_HEADERS = arch.h arp.h checksum.h conf.h dhcp.h dhcpv6.h flow.h \
> -	flow_table.h icmp.h inany.h isolation.h lineread.h log.h ndp.h \
> +	flow_table.h icmp.h inany.h iov.h isolation.h lineread.h log.h ndp.h \
>  	netlink.h packet.h passt.h pasta.h pcap.h pif.h port_fwd.h siphash.h \
>  	tap.h tcp.h tcp_conn.h tcp_splice.h udp.h util.h
>  HEADERS = $(PASST_HEADERS) seccomp.h
> diff --git a/iov.c b/iov.c
> new file mode 100644
> index 000000000000..73dd5cf25d0d
> --- /dev/null
> +++ b/iov.c
> @@ -0,0 +1,212 @@
> +// SPDX-License-Identifier: GPL-2.0-or-later
> +/*
> + * iov.h - helpers for using (partial) iovecs.
> + *
> + * Copyrigh (c) 2024 Red Hat

Typo: s/Copyrigh/Copyright/

AIUI, the "(c) 2024" has no real purpose, see
https://source.redhat.com/departments/legal/redhatintellectualproperty/intellectual_property_and_ip_litigation_wiki/copyright_notices_in_source_code

> + * Author: Laurent Vivier <lvivier@redhat.com>
> + *
> + * This file also contains code originally from QEMU include/qemu/iov.h
> + * and licensed under the following terms:
> + *
> + * Copyright (C) 2010 Red Hat, Inc.
> + *
> + * Author(s):
> + *  Amit Shah <amit.shah@redhat.com>
> + *  Michael Tokarev <mjt@tls.msk.ru>
> + *
> + * This work is licensed under the terms of the GNU GPL, version 2.  See
> + * the COPYING file in the top-level directory.
> + *
> + * Contributions after 2012-01-13 are licensed under the terms of the
> + * GNU GPL, version 2 or (at your option) any later version.

The description of the provenance of the code and its authorship is
useful.  I don't think the second copyright notice is useful in this
case, since it's also Red Hat, like the first.  Likewise, I don't
think the GPL invocation is useful, since we're not changing that
license.

> + */
> +#include <sys/socket.h>
> +
> +#include "util.h"
> +#include "iov.h"
> +
> +/**
> + * iov_from_buf - Copy data from a buffer to a scatter/gather
> + *                I/O vector (struct iovec) efficiently.
> + *
> + * @iov:       Pointer to the array of struct iovec describing the
> + *             scatter/gather I/O vector.

I feel like an IO vector is a common enough concept that we could just
say "IO vector" rather than this rather wordy description.

> + * @iov_cnt:   Number of elements in the iov array.
> + * @offset:    Byte offset in the iov array where copying should start.
> + * @buf:       Pointer to the source buffer containing the data to copy.
> + * @bytes:     Total number of bytes to copy from buf to iov.
> + *
> + * Returns:    The number of bytes successfully copied.
> + */
> +size_t iov_from_buf(const struct iovec *iov, unsigned int iov_cnt,
> +		    size_t offset, const void *buf, size_t bytes)
> +{
> +	if (__builtin_constant_p(bytes) && iov_cnt &&
> +		offset <= iov[0].iov_len && bytes <= iov[0].iov_len - offset) {
> +		memcpy((char *)iov[0].iov_base + offset, buf, bytes);
> +		return bytes;
> +	}
> +
> +	return iov_fill_from_buf(iov, iov_cnt, offset, buf, bytes);
> +}
> +
> +/**
> + * iov_to_buf - Copy data from a scatter/gather I/O vector (struct iovec) to
> + *		a buffer efficiently.
> + *
> + * @iov:       Pointer to the array of struct iovec describing the scatter/gather
> + *             I/O vector.
> + * @iov_cnt:   Number of elements in the iov array.
> + * @offset:    Offset within the first element of iov from where copying should start.
> + * @buf:       Pointer to the destination buffer where data will be copied.
> + * @bytes:     Total number of bytes to copy from iov to buf.
> + *
> + * Returns:    The number of bytes successfully copied.
> + */
> +size_t iov_to_buf(const struct iovec *iov, unsigned int iov_cnt,
> +		  size_t offset, void *buf, size_t bytes)
> +{
> +	if (__builtin_constant_p(bytes) && iov_cnt &&
> +		offset <= iov[0].iov_len && bytes <= iov[0].iov_len - offset) {
> +		memcpy(buf, (char *)iov[0].iov_base + offset, bytes);
> +		return bytes;
> +	}
> +
> +	return iov_fill_to_buf(iov, iov_cnt, offset, buf, bytes);
> +}
> +
> +/**
> + * iov_fill_from_buf - Copy data from a buffer to a scatter/gather
> + *                     I/O vector (struct iovec) until either all bytes
> + *                     are copied or all elements in the vector are filled.
> + *
> + * @iov:       Pointer to the array of struct iovec describing the scatter/gather
> + *             I/O vector.
> + * @iov_cnt:   Number of elements in the iov array.
> + * @offset:    Byte offset in the iov array where copying should start.
> + * @buf:       Pointer to the source buffer containing the data to copy.
> + * @bytes:     Total number of bytes to copy from buf to iov.
> + *
> + * Returns:    The total number of bytes successfully copied
> + *
> + */
> +size_t iov_fill_from_buf(const struct iovec *iov, unsigned int iov_cnt,
> +			 size_t offset, const void *buf, size_t bytes)

We could just open code this in iov_from_buf(), since I don't think we
ever have a reason to call it directly.

> +{
> +	unsigned int i;
> +	size_t copied;
> +
> +	/* skipping offset bytes in the iovec */
> +	for (i = 0; offset >= iov[i].iov_len && i < iov_cnt; i++)
> +		offset -= iov[i].iov_len;
> +
> +	/* copying data */
> +	for (copied = 0; copied < bytes && i < iov_cnt; i++) {
> +		size_t len = MIN(iov[i].iov_len - offset, bytes - copied);
> +
> +		memcpy((char *)iov[i].iov_base + offset, (char *)buf + copied,
> +		       len);
> +		copied += len;
> +		offset = 0;
> +	}
> +
> +	return copied;
> +}
> +
> +/**
> + * iov_fill_to_buf - Copy data from a scatter/gather I/O vector (struct iovec)
> + *                   to a buffer until either all bytes are copied or all
> + *                   elements in the vector are read.
> + *
> + * @iov:       Pointer to the array of struct iovec describing the
> + *             scatter/gather I/O vector.
> + * @iov_cnt:   Number of elements in the iov array.
> + * @offset:    Byte offset in the iov array where copying should start.
> + * @buf:       Pointer to the destination buffer where data will be copied.
> + * @bytes:     Total number of bytes to copy from iov to buf.
> + *
> + * Returns:    The total number of bytes successfully copied
> + */
> +size_t iov_fill_to_buf(const struct iovec *iov, unsigned int iov_cnt,
> +		       size_t offset, void *buf, size_t bytes)
> +{
> +	unsigned int i;
> +	size_t copied;
> +
> +	/* skipping offset bytes in the iovec */
> +	for (i = 0; offset >= iov[i].iov_len && i < iov_cnt; i++)
> +		offset -= iov[i].iov_len;
> +
> +	/* copying data */
> +	for (copied = 0; copied < bytes && i < iov_cnt; i++) {
> +		size_t len = MIN(iov[i].iov_len - offset, bytes - copied);
> +		memcpy((char *)buf + copied, (char *)iov[i].iov_base + offset,
> +		       len);
> +		copied += len;
> +		offset = 0;
> +	}
> +
> +	return copied;
> +}
> +
> +/**
> + * iov_size - Calculate the total size of a scatter/gather I/O vector
> + *            (struct iovec).
> + *
> + * @iov:       Pointer to the array of struct iovec describing the
> + *             scatter/gather I/O vector.
> + * @iov_cnt:   Number of elements in the iov array.
> + *
> + * Returns:    The total size in bytes.
> + */
> +size_t iov_size(const struct iovec *iov, const unsigned int iov_cnt)
> +{
> +	size_t len;
> +	unsigned int i;

Other order for these locals please (longest to shortest).

> +	for (i = 0, len = 0; i < iov_cnt; i++) {
> +		len += iov[i].iov_len;
> +	}

No braces here (passt style, again).

> +	return len;
> +}
> +
> +/**
> + * iov_copy - Copy data from one scatter/gather I/O vector (struct iovec) to
> + *            another.
> + *
> + * @dst_iov:      Pointer to the destination array of struct iovec describing
> + *                the scatter/gather I/O vector to copy to.
> + * @dst_iov_cnt:  Number of elements in the destination iov array.
> + * @iov:          Pointer to the source array of struct iovec describing
> + *                the scatter/gather I/O vector to copy from.
> + * @iov_cnt:      Number of elements in the source iov array.
> + * @offset:       Offset within the source iov from where copying should start.
> + * @bytes:        Total number of bytes to copy from iov to dst_iov.
> + *
> + * Returns:       The number of elements successfully copied to the destination
> + *                iov array.
> + */
> +unsigned iov_copy(struct iovec *dst_iov, unsigned int dst_iov_cnt,
> +		  const struct iovec *iov, unsigned int iov_cnt,
> +		  size_t offset, size_t bytes)
> +{
> +	unsigned int i, j;
> +	size_t len;
> +
> +	/* skipping offset bytes in the iovec */
> +	for (i = 0; offset >= iov[i].iov_len && i < iov_cnt; i++)
> +		offset -= iov[i].iov_len;
> +
> +	/* copying data */
> +	for (j = 0; i < iov_cnt && j < dst_iov_cnt && bytes; i++) {
> +		len = MIN(bytes, iov[i].iov_len - offset);
> +
> +		dst_iov[j].iov_base = (char *)iov[i].iov_base + offset;
> +		dst_iov[j].iov_len = len;
> +		j++;
> +		bytes -= len;
> +		offset = 0;
> +	}
> +
> +	return j;
> +}
> diff --git a/iov.h b/iov.h
> new file mode 100644
> index 000000000000..0153acca9e62
> --- /dev/null
> +++ b/iov.h
> @@ -0,0 +1,43 @@
> +// SPDX-License-Identifier: GPL-2.0-or-later
> +/*
> + * iov.c - helpers for using (partial) iovecs.
> + *
> + * Copyrigh (c) 2024 Red Hat

Same typo again.

> + * Author: Laurent Vivier <lvivier@redhat.com>
> + *
> + * This file also contains code originally from QEMU include/qemu/iov.h
> + * and licensed under the following terms:
> + *
> + * Copyright (C) 2010 Red Hat, Inc.
> + *
> + * Author(s):
> + *  Amit Shah <amit.shah@redhat.com>
> + *  Michael Tokarev <mjt@tls.msk.ru>
> + *
> + * This work is licensed under the terms of the GNU GPL, version 2.  See
> + * the COPYING file in the top-level directory.
> + *
> + * Contributions after 2012-01-13 are licensed under the terms of the
> + * GNU GPL, version 2 or (at your option) any later version.
> + */
> +
> +#ifndef IOVEC_H
> +#define IOVEC_H
> +
> +#include <unistd.h>
> +#include <string.h>
> +
> +size_t iov_fill_from_buf(const struct iovec *iov, unsigned int iov_cnt,
> +			 size_t offset, const void *buf, size_t bytes);
> +size_t iov_fill_to_buf(const struct iovec *iov, const unsigned int iov_cnt,
> +		       size_t offset, void *buf, size_t bytes);
> +size_t iov_from_buf(const struct iovec *iov, unsigned int iov_cnt,
> +                    size_t offset, const void *buf, size_t bytes);
> +size_t iov_to_buf(const struct iovec *iov, unsigned int iov_cnt,
> +                  size_t offset, void *buf, size_t bytes);
> +
> +size_t iov_size(const struct iovec *iov, const unsigned int iov_cnt);
> +unsigned iov_copy(struct iovec *dst_iov, unsigned int dst_iov_cnt,
> +		  const struct iovec *iov, unsigned int iov_cnt,
> +		  size_t offset, size_t bytes);
> +#endif /* IOVEC_H */

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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