At least on macOS 12.7.2, vmnet doesn't pad Ethernet frames, such as the
host's ARP replies, to the minimum size (60 bytes before the frame check
sequence) defined in IEEE Std 802.3-2022, so guests' Ethernet device
drivers may drop them with "frame too short" errors.

This patch calls eth_pad_short_frame() to add padding, as in net/tap.c
and net/slirp.c. Thanks to Bin Meng, Philippe Mathieu-Daudé, and Phil
Dennis-Jordan for reviewing earlier versions.

Signed-off-by: William Hooper <wsh@wshooper.org>
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2058
Reviewed-by: Phil Dennis-Jordan <phil@philjordan.eu>
---
 net/vmnet-common.m | 23 ++++++++++++++++++++---
 1 file changed, 20 insertions(+), 3 deletions(-)

diff --git a/net/vmnet-common.m b/net/vmnet-common.m
index 30c4e53c13..4b7e330c05 100644
--- a/net/vmnet-common.m
+++ b/net/vmnet-common.m
@@ -18,6 +18,7 @@
 #include "qemu/error-report.h"
 #include "qapi/error.h"
 #include "sysemu/runstate.h"
+#include "net/eth.h"
 
 #include <vmnet/vmnet.h>
 #include <dispatch/dispatch.h>
@@ -147,10 +148,26 @@ static int vmnet_read_packets(VmnetState *s)
  */
 static void vmnet_write_packets_to_qemu(VmnetState *s)
 {
+    uint8_t *pkt;
+    size_t pktsz;
+    uint8_t min_pkt[ETH_ZLEN];
+    size_t min_pktsz;
+    ssize_t size;
+
     while (s->packets_send_current_pos < s->packets_send_end_pos) {
-        ssize_t size = qemu_send_packet_async(&s->nc,
-                                      s->iov_buf[s->packets_send_current_pos].iov_base,
-                                      s->packets_buf[s->packets_send_current_pos].vm_pkt_size,
+        pkt = s->iov_buf[s->packets_send_current_pos].iov_base;
+        pktsz = s->packets_buf[s->packets_send_current_pos].vm_pkt_size;
+
+        if (net_peer_needs_padding(&s->nc)) {
+            min_pktsz = sizeof(min_pkt);
+
+            if (eth_pad_short_frame(min_pkt, &min_pktsz, pkt, pktsz)) {
+                pkt = min_pkt;
+                pktsz = min_pktsz;
+            }
+        }
+
+        size = qemu_send_packet_async(&s->nc, pkt, pktsz,
                                       vmnet_send_completed);
 
         if (size == 0) {

On Sat, Nov 2, 2024 at 1:56 PM William Hooper <wsh@wshooper.org> wrote:
> At least on macOS 12.7.2, vmnet doesn't pad Ethernet frames, such as the
> host's ARP replies, to the minimum size (60 bytes before the frame check
> sequence) defined in IEEE Std 802.3-2022, so guests' Ethernet device
> drivers may drop them with "frame too short" errors.
>
> This patch calls eth_pad_short_frame() to add padding, as in net/tap.c
> and net/slirp.c. Thanks to Bin Meng, Philippe Mathieu-Daudé, and Phil
> Dennis-Jordan for reviewing earlier versions.
>
> Signed-off-by: William Hooper <wsh@wshooper.org>
> Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2058
> Reviewed-by: Phil Dennis-Jordan <phil@philjordan.eu>
> ---
>  net/vmnet-common.m | 23 ++++++++++++++++++++---
>  1 file changed, 20 insertions(+), 3 deletions(-)
>
> diff --git a/net/vmnet-common.m b/net/vmnet-common.m
> index 30c4e53c13..4b7e330c05 100644
> --- a/net/vmnet-common.m
> +++ b/net/vmnet-common.m
> @@ -18,6 +18,7 @@
>  #include "qemu/error-report.h"
>  #include "qapi/error.h"
>  #include "sysemu/runstate.h"
> +#include "net/eth.h"
>
>  #include <vmnet/vmnet.h>
>  #include <dispatch/dispatch.h>
> @@ -147,10 +148,26 @@ static int vmnet_read_packets(VmnetState *s)
>   */
>  static void vmnet_write_packets_to_qemu(VmnetState *s)
>  {
> +    uint8_t *pkt;
> +    size_t pktsz;
> +    uint8_t min_pkt[ETH_ZLEN];
> +    size_t min_pktsz;
> +    ssize_t size;
> +
>      while (s->packets_send_current_pos < s->packets_send_end_pos) {
> -        ssize_t size = qemu_send_packet_async(&s->nc,
> -                                      s->iov_buf[s->packets_send_current_pos].iov_base,
> -                                      s->packets_buf[s->packets_send_current_pos].vm_pkt_size,
> +        pkt = s->iov_buf[s->packets_send_current_pos].iov_base;
> +        pktsz = s->packets_buf[s->packets_send_current_pos].vm_pkt_size;
> +
> +        if (net_peer_needs_padding(&s->nc)) {
> +            min_pktsz = sizeof(min_pkt);
> +
> +            if (eth_pad_short_frame(min_pkt, &min_pktsz, pkt, pktsz)) {
> +                pkt = min_pkt;
> +                pktsz = min_pktsz;
> +            }
> +        }
> +
> +        size = qemu_send_packet_async(&s->nc, pkt, pktsz,
>                                        vmnet_send_completed);
>
>          if (size == 0) {

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