Re: [PATCH 01/24] iov: add some functions to manage iovec

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

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On Fri, Feb 02, 2024 at 03:11:28PM +0100, Laurent Vivier wrote:
> Signed-off-by: Laurent Vivier <lvivier@redhat.com>
> ---
>  Makefile |  4 +--
>  iov.c    | 78 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++
>  iov.h    | 46 +++++++++++++++++++++++++++++++++
>  3 files changed, 126 insertions(+), 2 deletions(-)
>  create mode 100644 iov.c
>  create mode 100644 iov.h
> 
> diff --git a/Makefile b/Makefile
> index af4fa87e7e13..c1138fb91d26 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -47,7 +47,7 @@ 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
> +	util.c iov.c

I think we've been maintaining these in alphabetical order so far.

>  QRAP_SRCS = qrap.c
>  SRCS = $(PASST_SRCS) $(QRAP_SRCS)
>  
> @@ -56,7 +56,7 @@ 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 \
>  	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
> +	tap.h tcp.h tcp_conn.h tcp_splice.h udp.h util.h iov.h
>  HEADERS = $(PASST_HEADERS) seccomp.h
>  
>  C := \#include <linux/tcp.h>\nstruct tcp_info x = { .tcpi_snd_wnd = 0 };
> diff --git a/iov.c b/iov.c
> new file mode 100644
> index 000000000000..38a8e7566021
> --- /dev/null
> +++ b/iov.c
> @@ -0,0 +1,78 @@
> +// SPDX-License-Identifier: GPL-2.0-or-later

I believe we need an actual copyright / authorship notice as well as
the SPDX comment.


> +
> +/* some parts copied from QEMU util/iov.c */
> +
> +#include <sys/socket.h>
> +#include "util.h"
> +#include "iov.h"
> +

Function comments would be really helpful here.  It took me a while to
figure out what this does from just the name and implementation.

> +size_t iov_from_buf_full(const struct iovec *iov, unsigned int iov_cnt,
> +			 size_t offset, const void *buf, size_t bytes)
> +{
> +	size_t done;
> +	unsigned int i;

passt style is to order local declarations from longest to shortest.

> +	for (i = 0, done = 0; (offset || done < bytes) && i < iov_cnt; i++) {

Not immediately seeing why you need the 'offset ||' part of the condition.

> +		if (offset < iov[i].iov_len) {
> +			size_t len = MIN(iov[i].iov_len - offset, bytes - done);
> +			memcpy((char *)iov[i].iov_base + offset, (char *)buf + done, len);
> +			done += len;
> +			offset = 0;
> +		} else {
> +			offset -= iov[i].iov_len;
> +		}
> +	}
> +	return done;
> +}
> +
> +size_t iov_to_buf_full(const struct iovec *iov, const unsigned int iov_cnt,

const modifier on an int isn't very useful.

> +		       size_t offset, void *buf, size_t bytes)
> +{
> +	size_t done;
> +	unsigned int i;
> +	for (i = 0, done = 0; (offset || done < bytes) && i < iov_cnt; i++) {
> +		if (offset < iov[i].iov_len) {
> +			size_t len = MIN(iov[i].iov_len - offset, bytes - done);
> +			memcpy((char *)buf + done, (char *)iov[i].iov_base + offset, len);
> +			done += len;
> +			offset = 0;
> +		} else {
> +			offset -= iov[i].iov_len;
> +		}
> +	}
> +	return done;
> +}
> +
> +size_t iov_size(const struct iovec *iov, const unsigned int iov_cnt)
> +{
> +	size_t len;
> +	unsigned int i;
> +
> +	len = 0;

Can be an initialiser.

> +	for (i = 0; i < iov_cnt; i++) {
> +		len += iov[i].iov_len;
> +	}
> +	return len;
> +}
> +
> +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)
> +{
> +	size_t len;
> +	unsigned int i, j;
> +	for (i = 0, j = 0;
> +		 i < iov_cnt && j < dst_iov_cnt && (offset || bytes); i++) {
> +		if (offset >= iov[i].iov_len) {
> +			offset -= iov[i].iov_len;
> +			continue;
> +		}
> +		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;
> +}

Small concern about the interface to iov_copy().  If dst_iov_cnt <
iov_cnt and the chunk of the input iovec you want doesn't fit in the
destination it will silently truncate - you can't tell if this has
happened from the return value.  If the assumption is that dst_iov_cnt
>= iov_cnt, then there's not really any need to pass it.

> diff --git a/iov.h b/iov.h
> new file mode 100644
> index 000000000000..31fbf6d0e1cf
> --- /dev/null
> +++ b/iov.h
> @@ -0,0 +1,46 @@
> +// SPDX-License-Identifier: GPL-2.0-or-later
> +
> +/* some parts copied from QEMU include/qemu/iov.h */
> +
> +#ifndef IOVEC_H
> +#define IOVEC_H
> +
> +#include <unistd.h>
> +#include <string.h>
> +
> +size_t iov_from_buf_full(const struct iovec *iov, unsigned int iov_cnt,
> +			 size_t offset, const void *buf, size_t bytes);
> +size_t iov_to_buf_full(const struct iovec *iov, const unsigned int iov_cnt,
> +		       size_t offset, void *buf, size_t bytes);
> +
> +static inline 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;

Do you have an idea of how much difference this optimized path makes?

> +	} else {
> +		return iov_from_buf_full(iov, iov_cnt, offset, buf, bytes);
> +	}
> +}
> +
> +static inline size_t iov_to_buf(const struct iovec *iov,
> +				const 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;
> +	} else {
> +		return iov_to_buf_full(iov, iov_cnt, offset, buf, 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

Normal to put a /* IOVEC_H */ comment on the final #endif.

-- 
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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