[PATCH v4 8/8] trace-cmd: Use splice(2) for vsock sockets if available

[Slavomir Kaslev <kaslevs@vmware.com>]

Detect if splice(2) reading is supported for vsock sockets (Linux 4.20 and
later) and use it, or fallback to read/write-ing otherwise.

Signed-off-by: Slavomir Kaslev <kaslevs@vmware.com>
---
 tracecmd/trace-record.c | 59 ++++++++++++++++++++++++++++++++++++++---
 1 file changed, 56 insertions(+), 3 deletions(-)

diff --git a/tracecmd/trace-record.c b/tracecmd/trace-record.c
index 1622b5e..cd80462 100644
--- a/tracecmd/trace-record.c
+++ b/tracecmd/trace-record.c
@@ -2658,12 +2658,61 @@ static int open_vsock(unsigned int cid, unsigned int port)
 
 	return sd;
 }
+
+static int try_splice_read_vsock(void)
+{
+	int ret, sd, brass[2];
+
+	sd = socket(AF_VSOCK, SOCK_STREAM, 0);
+	if (sd < 0)
+		return sd;
+
+	ret = pipe(brass);
+	if (ret < 0)
+		goto out_close_sd;
+
+	/*
+	 * On kernels that don't support splice reading from vsock sockets
+	 * this will fail with EINVAL, or ENOTCONN otherwise.
+	 * Technically, it should never succeed but if it does, claim splice
+	 * reading is supported.
+	 */
+	ret = splice(sd, NULL, brass[1], NULL, 10, 0);
+	if (ret < 0)
+		ret = errno != EINVAL;
+	else
+		ret = 1;
+
+	close(brass[0]);
+	close(brass[1]);
+out_close_sd:
+	close(sd);
+	return ret;
+}
+
+static bool can_splice_read_vsock(void)
+{
+	static bool initialized, res;
+
+	if (initialized)
+		return res;
+
+	res = try_splice_read_vsock() > 0;
+	initialized = true;
+	return res;
+}
+
 #else
 static inline int open_vsock(unsigned int cid, unsigned int port)
 {
 	die("vsock is not supported");
 	return -1;
 }
+
+static bool can_splice_read_vsock(void)
+{
+	return false;
+}
 #endif
 
 static int do_accept(int sd)
@@ -2868,13 +2917,16 @@ create_recorder_instance(struct buffer_instance *instance, const char *file, int
 
 	if (instance->flags & BUFFER_FL_GUEST) {
 		int sd;
+		unsigned int flags;
 
 		sd = open_vsock(instance->cid, instance->client_ports[cpu]);
 		if (sd < 0)
 			die("Failed to connect to agent");
 
-		return tracecmd_create_recorder_virt(
-			file, cpu, recorder_flags | TRACECMD_RECORD_NOSPLICE, sd);
+		flags = recorder_flags;
+		if (!can_splice_read_vsock())
+			flags |= TRACECMD_RECORD_NOSPLICE;
+		return tracecmd_create_recorder_virt(file, cpu, flags, sd);
 	}
 
 	if (brass)
@@ -2928,7 +2980,8 @@ static int create_recorder(struct buffer_instance *instance, int cpu,
 
 		if (instance->flags & BUFFER_FL_AGENT) {
 			fd = do_accept(instance->fds[cpu]);
-			flags |= TRACECMD_RECORD_NOSPLICE;
+			if (!can_splice_read_vsock())
+				flags |= TRACECMD_RECORD_NOSPLICE;
 		} else {
 			fd = connect_port(host, instance->client_ports[cpu]);
 		}
-- 
2.19.1