[PATCH 0/4] RFC: Some cleanups to tap and tcp

[PATCH 0/4] RFC: Some cleanups to tap and tcp

[David Gibson]

This is a handful of simple cleanups which I made while investigating
https://bugs.passt.top/show_bug.cgi?id=41.  Note that these don't
themselves actually address that bug, they're just unrelated cleanups
that happen to be in adjacent code.

These are barely tested at all.  I've had some crises right before
going away that mean I haven't had a chance to plish these.  I'm
posting them so they're out there rather than waiting until I'm back
in two weeks.

These are based on my tap send unification series and the TCP socket
pool cleanup series.

David Gibson (4):
  tap: Don't pcap frames that didn't get sent
  tap: Eliminate goto from tap_handler()
  tcp: Remove 'recvmsg' goto from tcp_data_from_sock
  tcp: Remove 'zero_len' goto from tcp_data_from_sock

 tap.c | 49 +++++++++++++++++++++++++++----------------------
 tcp.c | 37 +++++++++++++++++--------------------
 2 files changed, 44 insertions(+), 42 deletions(-)

-- 
2.39.1

[PATCH 1/4] tap: Don't pcap frames that didn't get sent

[David Gibson]

In tap_send_frames() we send a number of frames to the tap device, then
also write them to the pcap capture file (if configured).  However the tap
send can partially fail (short write()s or similar), meaning that some
of the requested frames weren't actually sent, but we still write those
frames to the capture file.

We do give a debug message in this case, but it's misleading to add frames
that we know weren't sent to the capture file.  Rework to avoid this.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
---
 tap.c | 30 ++++++++++++++++++++----------
 1 file changed, 20 insertions(+), 10 deletions(-)

diff --git a/tap.c b/tap.c
index af9bc15..dd22490 100644
--- a/tap.c
+++ b/tap.c
@@ -309,10 +309,12 @@ void tap_icmp6_send(const struct ctx *c,
  * @iov:	Array of buffers, each containing one frame
  * @n:		Number of buffers/frames in @iov
  *
+ * Returns: number of frames successfully sent
+ *
  * #syscalls:pasta write
  */
-static void tap_send_frames_pasta(struct ctx *c,
-				  const struct iovec *iov, size_t n)
+static size_t tap_send_frames_pasta(struct ctx *c,
+				    const struct iovec *iov, size_t n)
 {
 	size_t i;
 
@@ -324,6 +326,8 @@ static void tap_send_frames_pasta(struct ctx *c,
 			i--;
 		}
 	}
+
+	return n;
 }
 
 /**
@@ -356,10 +360,12 @@ static void tap_send_remainder(const struct ctx *c, const struct iovec *iov,
  * @iov:	Array of buffers, each containing one frame
  * @n:		Number of buffers/frames in @iov
  *
+ * Returns: number of frames successfully sent
+ *
  * #syscalls:passt sendmsg
  */
-static void tap_send_frames_passt(const struct ctx *c,
-				  const struct iovec *iov, size_t n)
+static size_t tap_send_frames_passt(const struct ctx *c,
+				    const struct iovec *iov, size_t n)
 {
 	struct msghdr mh = {
 		.msg_iov = (void *)iov,
@@ -370,7 +376,7 @@ static void tap_send_frames_passt(const struct ctx *c,
 
 	sent = sendmsg(c->fd_tap, &mh, MSG_NOSIGNAL | MSG_DONTWAIT);
 	if (sent < 0)
-		return;
+		return 0;
 
 	/* Check for any partial frames due to short send */
 	for (i = 0; i < n; i++) {
@@ -385,8 +391,7 @@ static void tap_send_frames_passt(const struct ctx *c,
 		i++;
 	}
 
-	if (i < n)
-		debug("tap: dropped %lu frames due to short send", n - i);
+	return i;
 }
 
 /**
@@ -397,15 +402,20 @@ static void tap_send_frames_passt(const struct ctx *c,
  */
 void tap_send_frames(struct ctx *c, const struct iovec *iov, size_t n)
 {
+	size_t m;
+
 	if (!n)
 		return;
 
 	if (c->mode == MODE_PASST)
-		tap_send_frames_passt(c, iov, n);
+		m = tap_send_frames_passt(c, iov, n);
 	else
-		tap_send_frames_pasta(c, iov, n);
+		m = tap_send_frames_pasta(c, iov, n);
+
+	if (m < n)
+		debug("tap: dropped %lu frames of %lu due to short send", n - m, n);
 
-	pcap_multiple(iov, n, c->mode == MODE_PASST ? sizeof(uint32_t) : 0);
+	pcap_multiple(iov, m, c->mode == MODE_PASST ? sizeof(uint32_t) : 0);
 }
 
 /**
-- 
@@ -309,10 +309,12 @@ void tap_icmp6_send(const struct ctx *c,
  * @iov:	Array of buffers, each containing one frame
  * @n:		Number of buffers/frames in @iov
  *
+ * Returns: number of frames successfully sent
+ *
  * #syscalls:pasta write
  */
-static void tap_send_frames_pasta(struct ctx *c,
-				  const struct iovec *iov, size_t n)
+static size_t tap_send_frames_pasta(struct ctx *c,
+				    const struct iovec *iov, size_t n)
 {
 	size_t i;
 
@@ -324,6 +326,8 @@ static void tap_send_frames_pasta(struct ctx *c,
 			i--;
 		}
 	}
+
+	return n;
 }
 
 /**
@@ -356,10 +360,12 @@ static void tap_send_remainder(const struct ctx *c, const struct iovec *iov,
  * @iov:	Array of buffers, each containing one frame
  * @n:		Number of buffers/frames in @iov
  *
+ * Returns: number of frames successfully sent
+ *
  * #syscalls:passt sendmsg
  */
-static void tap_send_frames_passt(const struct ctx *c,
-				  const struct iovec *iov, size_t n)
+static size_t tap_send_frames_passt(const struct ctx *c,
+				    const struct iovec *iov, size_t n)
 {
 	struct msghdr mh = {
 		.msg_iov = (void *)iov,
@@ -370,7 +376,7 @@ static void tap_send_frames_passt(const struct ctx *c,
 
 	sent = sendmsg(c->fd_tap, &mh, MSG_NOSIGNAL | MSG_DONTWAIT);
 	if (sent < 0)
-		return;
+		return 0;
 
 	/* Check for any partial frames due to short send */
 	for (i = 0; i < n; i++) {
@@ -385,8 +391,7 @@ static void tap_send_frames_passt(const struct ctx *c,
 		i++;
 	}
 
-	if (i < n)
-		debug("tap: dropped %lu frames due to short send", n - i);
+	return i;
 }
 
 /**
@@ -397,15 +402,20 @@ static void tap_send_frames_passt(const struct ctx *c,
  */
 void tap_send_frames(struct ctx *c, const struct iovec *iov, size_t n)
 {
+	size_t m;
+
 	if (!n)
 		return;
 
 	if (c->mode == MODE_PASST)
-		tap_send_frames_passt(c, iov, n);
+		m = tap_send_frames_passt(c, iov, n);
 	else
-		tap_send_frames_pasta(c, iov, n);
+		m = tap_send_frames_pasta(c, iov, n);
+
+	if (m < n)
+		debug("tap: dropped %lu frames of %lu due to short send", n - m, n);
 
-	pcap_multiple(iov, n, c->mode == MODE_PASST ? sizeof(uint32_t) : 0);
+	pcap_multiple(iov, m, c->mode == MODE_PASST ? sizeof(uint32_t) : 0);
 }
 
 /**
-- 
2.39.1

[PATCH 2/4] tap: Eliminate goto from tap_handler()

[David Gibson]

The goto here really doesn't improve clarity or brevity at all.  Use a
clearer construct.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
---
 tap.c | 19 +++++++------------
 1 file changed, 7 insertions(+), 12 deletions(-)

diff --git a/tap.c b/tap.c
index dd22490..757fb86 100644
--- a/tap.c
+++ b/tap.c
@@ -1238,18 +1238,13 @@ void tap_handler(struct ctx *c, int fd, uint32_t events,
 	}
 
 	if ((c->mode == MODE_PASST && tap_handler_passt(c, now)) ||
-	    (c->mode == MODE_PASTA && tap_handler_pasta(c, now)))
-		goto reinit;
-
-	if (events & (EPOLLRDHUP | EPOLLHUP | EPOLLERR))
-		goto reinit;
+	    (c->mode == MODE_PASTA && tap_handler_pasta(c, now)) ||
+	    (events & (EPOLLRDHUP | EPOLLHUP | EPOLLERR))) {
+		if (c->one_off) {
+			info("Client closed connection, exiting");
+			exit(EXIT_SUCCESS);
+		}
 
-	return;
-reinit:
-	if (c->one_off) {
-		info("Client closed connection, exiting");
-		exit(EXIT_SUCCESS);
+		tap_sock_init(c);
 	}
-
-	tap_sock_init(c);
 }
-- 
@@ -1238,18 +1238,13 @@ void tap_handler(struct ctx *c, int fd, uint32_t events,
 	}
 
 	if ((c->mode == MODE_PASST && tap_handler_passt(c, now)) ||
-	    (c->mode == MODE_PASTA && tap_handler_pasta(c, now)))
-		goto reinit;
-
-	if (events & (EPOLLRDHUP | EPOLLHUP | EPOLLERR))
-		goto reinit;
+	    (c->mode == MODE_PASTA && tap_handler_pasta(c, now)) ||
+	    (events & (EPOLLRDHUP | EPOLLHUP | EPOLLERR))) {
+		if (c->one_off) {
+			info("Client closed connection, exiting");
+			exit(EXIT_SUCCESS);
+		}
 
-	return;
-reinit:
-	if (c->one_off) {
-		info("Client closed connection, exiting");
-		exit(EXIT_SUCCESS);
+		tap_sock_init(c);
 	}
-
-	tap_sock_init(c);
 }
-- 
2.39.1

[PATCH 3/4] tcp: Remove 'recvmsg' goto from tcp_data_from_sock

[David Gibson]

This goto can be handled just as simply and more clearly with a do while.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
---
 tcp.c | 11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)

diff --git a/tcp.c b/tcp.c
index f0085e3..f7228d1 100644
--- a/tcp.c
+++ b/tcp.c
@@ -2158,13 +2158,12 @@ static int tcp_data_from_sock(struct ctx *c, struct tcp_tap_conn *conn)
 		iov_sock[fill_bufs].iov_len = iov_rem;
 
 	/* Receive into buffers, don't dequeue until acknowledged by guest. */
-recvmsg:
-	len = recvmsg(s, &mh_sock, MSG_PEEK);
-	if (len < 0) {
-		if (errno == EINTR)
-			goto recvmsg;
+	do {
+		len = recvmsg(s, &mh_sock, MSG_PEEK);
+	} while (len < 0 && errno == EINTR);
+
+	if (len < 0)
 		goto err;
-	}
 
 	if (!len)
 		goto zero_len;
-- 
@@ -2158,13 +2158,12 @@ static int tcp_data_from_sock(struct ctx *c, struct tcp_tap_conn *conn)
 		iov_sock[fill_bufs].iov_len = iov_rem;
 
 	/* Receive into buffers, don't dequeue until acknowledged by guest. */
-recvmsg:
-	len = recvmsg(s, &mh_sock, MSG_PEEK);
-	if (len < 0) {
-		if (errno == EINTR)
-			goto recvmsg;
+	do {
+		len = recvmsg(s, &mh_sock, MSG_PEEK);
+	} while (len < 0 && errno == EINTR);
+
+	if (len < 0)
 		goto err;
-	}
 
 	if (!len)
 		goto zero_len;
-- 
2.39.1

[PATCH 4/4] tcp: Remove 'zero_len' goto from tcp_data_from_sock

[David Gibson]

This goto exists purely to move this exception case out of line.  Although
that does make the "normal" path a little clearer, it comes at the cost of
not knowing how where control will flow after jumping to the zero_len
label.  The exceptional case isn't that long, so just put it inline.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
---
 tcp.c | 26 ++++++++++++--------------
 1 file changed, 12 insertions(+), 14 deletions(-)

diff --git a/tcp.c b/tcp.c
index f7228d1..4ae5ed5 100644
--- a/tcp.c
+++ b/tcp.c
@@ -2165,8 +2165,18 @@ static int tcp_data_from_sock(struct ctx *c, struct tcp_tap_conn *conn)
 	if (len < 0)
 		goto err;
 
-	if (!len)
-		goto zero_len;
+	if (!len) {
+		if ((conn->events & (SOCK_FIN_RCVD | TAP_FIN_SENT)) == SOCK_FIN_RCVD) {
+			if ((ret = tcp_send_flag(c, conn, FIN | ACK))) {
+				tcp_rst(c, conn);
+				return ret;
+			}
+
+			conn_event(c, conn, TAP_FIN_SENT);
+		}
+
+		return 0;
+	}
 
 	sendlen = len - already_sent;
 	if (sendlen <= 0) {
@@ -2205,18 +2215,6 @@ err:
 	}
 
 	return ret;
-
-zero_len:
-	if ((conn->events & (SOCK_FIN_RCVD | TAP_FIN_SENT)) == SOCK_FIN_RCVD) {
-		if ((ret = tcp_send_flag(c, conn, FIN | ACK))) {
-			tcp_rst(c, conn);
-			return ret;
-		}
-
-		conn_event(c, conn, TAP_FIN_SENT);
-	}
-
-	return 0;
 }
 
 /**
-- 
@@ -2165,8 +2165,18 @@ static int tcp_data_from_sock(struct ctx *c, struct tcp_tap_conn *conn)
 	if (len < 0)
 		goto err;
 
-	if (!len)
-		goto zero_len;
+	if (!len) {
+		if ((conn->events & (SOCK_FIN_RCVD | TAP_FIN_SENT)) == SOCK_FIN_RCVD) {
+			if ((ret = tcp_send_flag(c, conn, FIN | ACK))) {
+				tcp_rst(c, conn);
+				return ret;
+			}
+
+			conn_event(c, conn, TAP_FIN_SENT);
+		}
+
+		return 0;
+	}
 
 	sendlen = len - already_sent;
 	if (sendlen <= 0) {
@@ -2205,18 +2215,6 @@ err:
 	}
 
 	return ret;
-
-zero_len:
-	if ((conn->events & (SOCK_FIN_RCVD | TAP_FIN_SENT)) == SOCK_FIN_RCVD) {
-		if ((ret = tcp_send_flag(c, conn, FIN | ACK))) {
-			tcp_rst(c, conn);
-			return ret;
-		}
-
-		conn_event(c, conn, TAP_FIN_SENT);
-	}
-
-	return 0;
 }
 
 /**
-- 
2.39.1

Re: [PATCH 1/4] tap: Don't pcap frames that didn't get sent

[Stefano Brivio]

On Fri, 27 Jan 2023 16:11:07 +1100
David Gibson <david@gibson.dropbear.id.au> wrote:

> In tap_send_frames() we send a number of frames to the tap device, then
> also write them to the pcap capture file (if configured).  However the tap
> send can partially fail (short write()s or similar), meaning that some
> of the requested frames weren't actually sent, but we still write those
> frames to the capture file.
> 
> We do give a debug message in this case, but it's misleading to add frames
> that we know weren't sent to the capture file.  Rework to avoid this.

To be really "correct", I guess we should also truncate messages in
captures if they were sent partially, by returning the number of bytes
sent from tap_send_frames_{pasta,passt}() and then modifying the
argument to pcap_frame() in the pcap_multiple() loop.

This is relevant because, if a packet has a checksum, we could consider
it lost while checking captures.

Still, it's a vast improvement on the original, so I would apply this
even like it is -- except for two nits, below:

> Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> ---
>  tap.c | 30 ++++++++++++++++++++----------
>  1 file changed, 20 insertions(+), 10 deletions(-)
> 
> diff --git a/tap.c b/tap.c
> index af9bc15..dd22490 100644
> --- a/tap.c
> +++ b/tap.c
> @@ -309,10 +309,12 @@ void tap_icmp6_send(const struct ctx *c,
>   * @iov:	Array of buffers, each containing one frame
>   * @n:		Number of buffers/frames in @iov
>   *
> + * Returns: number of frames successfully sent

For consistency: "Return:" -- I see now that one slipped through in
pcap_frame(). I can "fix" this on merge or in a follow-up patch, too.

> + *
>   * #syscalls:pasta write
>   */
> -static void tap_send_frames_pasta(struct ctx *c,
> -				  const struct iovec *iov, size_t n)
> +static size_t tap_send_frames_pasta(struct ctx *c,
> +				    const struct iovec *iov, size_t n)
>  {
>  	size_t i;
>  
> @@ -324,6 +326,8 @@ static void tap_send_frames_pasta(struct ctx *c,
>  			i--;
>  		}
>  	}
> +
> +	return n;
>  }
>  
>  /**
> @@ -356,10 +360,12 @@ static void tap_send_remainder(const struct ctx *c, const struct iovec *iov,
>   * @iov:	Array of buffers, each containing one frame
>   * @n:		Number of buffers/frames in @iov
>   *
> + * Returns: number of frames successfully sent

Same here.

-- 
Stefano

Re: [PATCH 3/4] tcp: Remove 'recvmsg' goto from tcp_data_from_sock

[Stefano Brivio]

Nit:

On Fri, 27 Jan 2023 16:11:09 +1100
David Gibson <david@gibson.dropbear.id.au> wrote:

> This goto can be handled just as simply and more clearly with a do while.
> 
> Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> ---
>  tcp.c | 11 +++++------
>  1 file changed, 5 insertions(+), 6 deletions(-)
> 
> diff --git a/tcp.c b/tcp.c
> index f0085e3..f7228d1 100644
> --- a/tcp.c
> +++ b/tcp.c
> @@ -2158,13 +2158,12 @@ static int tcp_data_from_sock(struct ctx *c, struct tcp_tap_conn *conn)
>  		iov_sock[fill_bufs].iov_len = iov_rem;
>  
>  	/* Receive into buffers, don't dequeue until acknowledged by guest. */
> -recvmsg:
> -	len = recvmsg(s, &mh_sock, MSG_PEEK);
> -	if (len < 0) {
> -		if (errno == EINTR)
> -			goto recvmsg;
> +	do {
> +		len = recvmsg(s, &mh_sock, MSG_PEEK);
> +	} while (len < 0 && errno == EINTR);

For consistency, we shouldn't use curly brackets if there's a single
line in the loop body (only other occurrence: pasta_wait_for_ns()).

I don't have a strong preference here and I can also fix it up on
merge, by the way.

-- 
Stefano

Re: [PATCH 3/4] tcp: Remove 'recvmsg' goto from tcp_data_from_sock

[David Gibson]

[-- Attachment #1: Type: text/plain, Size: 1494 bytes --]

On Wed, Feb 15, 2023 at 01:17:33PM +0100, Stefano Brivio wrote:
> Nit:
> 
> On Fri, 27 Jan 2023 16:11:09 +1100
> David Gibson <david@gibson.dropbear.id.au> wrote:
> 
> > This goto can be handled just as simply and more clearly with a do while.
> > 
> > Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> > ---
> >  tcp.c | 11 +++++------
> >  1 file changed, 5 insertions(+), 6 deletions(-)
> > 
> > diff --git a/tcp.c b/tcp.c
> > index f0085e3..f7228d1 100644
> > --- a/tcp.c
> > +++ b/tcp.c
> > @@ -2158,13 +2158,12 @@ static int tcp_data_from_sock(struct ctx *c, struct tcp_tap_conn *conn)
> >  		iov_sock[fill_bufs].iov_len = iov_rem;
> >  
> >  	/* Receive into buffers, don't dequeue until acknowledged by guest. */
> > -recvmsg:
> > -	len = recvmsg(s, &mh_sock, MSG_PEEK);
> > -	if (len < 0) {
> > -		if (errno == EINTR)
> > -			goto recvmsg;
> > +	do {
> > +		len = recvmsg(s, &mh_sock, MSG_PEEK);
> > +	} while (len < 0 && errno == EINTR);
> 
> For consistency, we shouldn't use curly brackets if there's a single
> line in the loop body (only other occurrence: pasta_wait_for_ns()).

Huh... I never even knew the braces were optional for do while.

> I don't have a strong preference here and I can also fix it up on
> merge, by the way.
> 

-- 
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

[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 833 bytes --]

Re: [PATCH 1/4] tap: Don't pcap frames that didn't get sent

[David Gibson]

[-- Attachment #1: Type: text/plain, Size: 2930 bytes --]

On Wed, Feb 15, 2023 at 01:17:25PM +0100, Stefano Brivio wrote:
> On Fri, 27 Jan 2023 16:11:07 +1100
> David Gibson <david@gibson.dropbear.id.au> wrote:
> 
> > In tap_send_frames() we send a number of frames to the tap device, then
> > also write them to the pcap capture file (if configured).  However the tap
> > send can partially fail (short write()s or similar), meaning that some
> > of the requested frames weren't actually sent, but we still write those
> > frames to the capture file.
> > 
> > We do give a debug message in this case, but it's misleading to add frames
> > that we know weren't sent to the capture file.  Rework to avoid this.
> 
> To be really "correct", I guess we should also truncate messages in
> captures if they were sent partially, by returning the number of bytes
> sent from tap_send_frames_{pasta,passt}() and then modifying the
> argument to pcap_frame() in the pcap_multiple() loop.

True.. that only applies for the pasta case, though.  For passt we
always send a whole frame, or the stream would get out of sync.

> This is relevant because, if a packet has a checksum, we could consider
> it lost while checking captures.
> 
> Still, it's a vast improvement on the original, so I would apply this
> even like it is -- except for two nits, below:
> 
> > Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> > ---
> >  tap.c | 30 ++++++++++++++++++++----------
> >  1 file changed, 20 insertions(+), 10 deletions(-)
> > 
> > diff --git a/tap.c b/tap.c
> > index af9bc15..dd22490 100644
> > --- a/tap.c
> > +++ b/tap.c
> > @@ -309,10 +309,12 @@ void tap_icmp6_send(const struct ctx *c,
> >   * @iov:	Array of buffers, each containing one frame
> >   * @n:		Number of buffers/frames in @iov
> >   *
> > + * Returns: number of frames successfully sent
> 
> For consistency: "Return:" -- I see now that one slipped through in
> pcap_frame(). I can "fix" this on merge or in a follow-up patch, too.
> 
> > + *
> >   * #syscalls:pasta write
> >   */
> > -static void tap_send_frames_pasta(struct ctx *c,
> > -				  const struct iovec *iov, size_t n)
> > +static size_t tap_send_frames_pasta(struct ctx *c,
> > +				    const struct iovec *iov, size_t n)
> >  {
> >  	size_t i;
> >  
> > @@ -324,6 +326,8 @@ static void tap_send_frames_pasta(struct ctx *c,
> >  			i--;
> >  		}
> >  	}
> > +
> > +	return n;
> >  }
> >  
> >  /**
> > @@ -356,10 +360,12 @@ static void tap_send_remainder(const struct ctx *c, const struct iovec *iov,
> >   * @iov:	Array of buffers, each containing one frame
> >   * @n:		Number of buffers/frames in @iov
> >   *
> > + * Returns: number of frames successfully sent
> 
> Same here.

Fixed.

-- 
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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Re: [PATCH 1/4] tap: Don't pcap frames that didn't get sent

[Stefano Brivio]

On Thu, 16 Feb 2023 16:20:44 +1100
David Gibson <david@gibson.dropbear.id.au> wrote:

> On Wed, Feb 15, 2023 at 01:17:25PM +0100, Stefano Brivio wrote:
> > On Fri, 27 Jan 2023 16:11:07 +1100
> > David Gibson <david@gibson.dropbear.id.au> wrote:
> >   
> > > In tap_send_frames() we send a number of frames to the tap device, then
> > > also write them to the pcap capture file (if configured).  However the tap
> > > send can partially fail (short write()s or similar), meaning that some
> > > of the requested frames weren't actually sent, but we still write those
> > > frames to the capture file.
> > > 
> > > We do give a debug message in this case, but it's misleading to add frames
> > > that we know weren't sent to the capture file.  Rework to avoid this.  
> > 
> > To be really "correct", I guess we should also truncate messages in
> > captures if they were sent partially, by returning the number of bytes
> > sent from tap_send_frames_{pasta,passt}() and then modifying the
> > argument to pcap_frame() in the pcap_multiple() loop.  
> 
> True.. that only applies for the pasta case, though.  For passt we
> always send a whole frame, or the stream would get out of sync.

Actually, I was thinking of the passt case too: the send() in
tap_send_remainder() might fail -- but then, contrary to what I wrote,
there's nothing to truncate, because the socket back-end in QEMU
doesn't deliver partial frames either. So this patch is actually
correct in that regard.

Unrelated: given that QEMU is going to have a "reconnect" option:
  http://patchwork.ozlabs.org/project/qemu-devel/patch/20230119101645.2001683-1-lvivier@redhat.com/

perhaps, if tap_send_remainder() fails, we should now close the socket
to give the guest the best chances to recover? Compared to terminating
the process, this has the advantage of keeping the whole state.

-- 
Stefano