Support logging xen-guest console

diff --git a/hw/xen_console.c b/hw/xen_console.c
index c6c8163..ac3208d 100644
--- a/hw/xen_console.c
+++ b/hw/xen_console.c
@@ -36,6 +36,8 @@ 
 #include "qemu-char.h"
 #include "xen_backend.h"
 
+static int log_guest = 0;
+
 struct buffer {
     uint8_t *data;
     size_t consumed;
@@ -52,8 +54,24 @@  struct XenConsole {
     void              *sring;
     CharDriverState   *chr;
     int               backlog;
+    int               log_fd;
 };
 
+static int write_all(int fd, const char* buf, size_t len)
+{
+    while (len) {
+        ssize_t ret = write(fd, buf, len);
+        if (ret == -1 && errno == EINTR)
+            continue;
+        if (ret <= 0)
+            return -1;
+        len -= ret;
+        buf += ret;
+    }
+
+    return 0;
+}
+
 static void buffer_append(struct XenConsole *con)
 {
     struct buffer *buffer = &con->buffer;
@@ -81,6 +99,14 @@  static void buffer_append(struct XenConsole *con)
     intf->out_cons = cons;
     xen_be_send_notify(&con->xendev);
 
+    if (con->log_fd != -1) {
+        int logret;
+        logret = write_all(con->log_fd, buffer->data + buffer->size - size, size);
+        if (logret < 0)
+            xen_be_printf(&con->xendev, 1, "Write to log failed on domain %d: %d (%s)\n",
+                      con->xendev.dom, errno, strerror(errno));
+     }
+
     if (buffer->max_capacity &&
 	buffer->size > buffer->max_capacity) {
 	/* Discard the middle of the data. */
@@ -174,12 +200,36 @@  static void xencons_send(struct XenConsole *con)
     }
 }
 
+static int create_domain_log(struct XenConsole *con)
+{
+    char *logfile;
+    char *path, *domname;
+    int fd;
+
+    path = xs_get_domain_path(xenstore, con->xendev.dom);
+    domname = xenstore_read_str(path, "name");
+    free(path);
+    if (!domname)
+        return -1;
+
+    asprintf(logfile, "/var/log/xen/console/guest-%s.log", domname);
+    qemu_free(domname);
+
+    fd = open(logfile, O_WRONLY|O_CREAT|O_APPEND, 0644);
+    free(logfile);
+    if (fd == -1)
+        xen_be_printf(&con->xendev, 1,  "Failed to open log %s: %d (%s)", logfile, errno, strerror(errno));
+
+    return fd;
+}
+
 /* -------------------------------------------------------------------- */
 
 static int con_init(struct XenDevice *xendev)
 {
     struct XenConsole *con = container_of(xendev, struct XenConsole, xendev);
     char *type, *dom;
+    char *logenv = NULL;
 
     /* setup */
     dom = xs_get_domain_path(xenstore, con->xendev.dom);
@@ -198,6 +248,10 @@  static int con_init(struct XenDevice *xendev)
     else
         con->chr = serial_hds[con->xendev.dev];
 
+    logenv = getenv("XENCONSOLED_TRACE");
+    if (logenv != NULL && !strcmp(logenv, "guest")) {
+        log_guest = 1;
+    }
     return 0;
 }
 
@@ -230,6 +284,9 @@  static int con_connect(struct XenDevice *xendev)
 		  con->xendev.remote_port,
 		  con->xendev.local_port,
 		  con->buffer.max_capacity);
+    con->log_fd = -1;
+    if (log_guest)
+         con->log_fd = create_domain_log(con);
     return 0;
 }
 
@@ -245,6 +302,12 @@  static void con_disconnect(struct XenDevice *xendev)
 	munmap(con->sring, XC_PAGE_SIZE);
 	con->sring = NULL;
     }
+
+    if (con->log_fd != -1) {
+        close(con->log_fd);
+        con->log_fd = -1;
+    }
+
 }
 
 static void con_event(struct XenDevice *xendev)