virtio/net/unixgram: Fix SO_SNDBUF size on macOS

src/devices/src/virtio/net/mod.rs

@@ -19,16 +19,13 @@ mod unixgram;
 mod unixstream;
 mod worker;
 
-fn vnet_hdr_len() -> usize {
-    mem::size_of::<virtio_net_hdr_v1>()
-}
+// https://docs.oasis-open.org/virtio/virtio/v1.1/csprd01/virtio-v1.1-csprd01.html#x1-2050006
+const VNET_HDR_LEN: usize = mem::size_of::<virtio_net_hdr_v1>();
 
 // This initializes to all 0 the virtio_net_hdr part of a buf and return the length of the header
-// https://docs.oasis-open.org/virtio/virtio/v1.1/csprd01/virtio-v1.1-csprd01.html#x1-2050006
 fn write_virtio_net_hdr(buf: &mut [u8]) -> usize {
-    let len = vnet_hdr_len();
-    buf[0..len].fill(0);
-    len
+    buf[0..VNET_HDR_LEN].fill(0);
+    VNET_HDR_LEN
 }
 
 pub use self::device::Net;

src/devices/src/virtio/net/unixgram.rs

@@ -9,8 +9,22 @@ use std::path::PathBuf;
 
 use super::backend::{ConnectError, NetBackend, ReadError, WriteError};
 use super::write_virtio_net_hdr;
+#[cfg(target_os = "macos")]
+use super::{MAX_BUFFER_SIZE, VNET_HDR_LEN};
 
 const VFKIT_MAGIC: [u8; 4] = *b"VFKT";
+const DEFAULT_SOCKET_BUF_SIZE: usize = 7 * 1024 * 1024;
+
+// On macOS, with UNIX datagram sockets the send buffer is not used for queuing;
+// it determines the maximum frame size.
+// https://github.com/apple-oss-distributions/xnu/blob/f6217f891ac0bb64f3d375211650a4c1ff8ca1ea/bsd/kern/uipc_usrreq.c#L953
+#[cfg(target_os = "macos")]
+const SOCKET_SNDBUF: usize = MAX_BUFFER_SIZE - VNET_HDR_LEN;
+
+#[cfg(not(target_os = "macos"))]
+const SOCKET_SNDBUF: usize = DEFAULT_SOCKET_BUF_SIZE;
+
+const SOCKET_RCVBUF: usize = DEFAULT_SOCKET_BUF_SIZE;
 
 pub struct Unixgram {
     fd: OwnedFd,
@@ -77,11 +91,11 @@ impl Unixgram {
                 .map_err(ConnectError::SendingMagic)?;
         }
 
-        if let Err(e) = setsockopt(&fd, sockopt::SndBuf, &(7 * 1024 * 1024)) {
-            log::warn!("Failed to increase SO_SNDBUF (performance may be decreased): {e}");
+        if let Err(e) = setsockopt(&fd, sockopt::SndBuf, &SOCKET_SNDBUF) {
+            log::warn!("Failed to set SO_SNDBUF: {e}");
         }
-        if let Err(e) = setsockopt(&fd, sockopt::RcvBuf, &(7 * 1024 * 1024)) {
-            log::warn!("Failed to increase SO_SNDBUF (performance may be decreased): {e}");
+        if let Err(e) = setsockopt(&fd, sockopt::RcvBuf, &SOCKET_RCVBUF) {
+            log::warn!("Failed to set SO_RCVBUF: {e}");
         }
 
         log::debug!(

src/devices/src/virtio/net/worker.rs

@@ -8,7 +8,7 @@ use crate::virtio::{DeviceQueue, InterruptTransport};
 
 use super::backend::{NetBackend, ReadError, WriteError};
 use super::device::{FrontendError, RxError, TxError, VirtioNetBackend};
-use super::vnet_hdr_len;
+use super::VNET_HDR_LEN;
 
 use std::os::fd::{AsRawFd, FromRawFd, OwnedFd};
 use std::thread;
@@ -202,7 +202,7 @@ impl NetWorker {
     pub(crate) fn process_backend_socket_writeable(&mut self) {
         match self
             .backend
-            .try_finish_write(vnet_hdr_len(), &self.tx_frame_buf[..self.tx_frame_len])
+            .try_finish_write(VNET_HDR_LEN, &self.tx_frame_buf[..self.tx_frame_len])
         {
             Ok(()) => self.process_tx_loop(),
             Err(WriteError::PartialWrite | WriteError::NothingWritten) => {}
@@ -275,7 +275,7 @@ impl NetWorker {
         if self.backend.has_unfinished_write()
             && self
                 .backend
-                .try_finish_write(vnet_hdr_len(), &self.tx_frame_buf[..self.tx_frame_len])
+                .try_finish_write(VNET_HDR_LEN, &self.tx_frame_buf[..self.tx_frame_len])
                 .is_err()
         {
             log::trace!("Cannot process tx because of unfinished partial write!");
@@ -321,7 +321,7 @@ impl NetWorker {
             self.tx_frame_len = read_count;
             match self
                 .backend
-                .write_frame(vnet_hdr_len(), &mut self.tx_frame_buf[..read_count])
+                .write_frame(VNET_HDR_LEN, &mut self.tx_frame_buf[..read_count])
             {
                 Ok(()) => {
                     self.tx_frame_len = 0;