Add boot option to allow console I/O to be placed into ring buffer

This option allows console I/O to be placed into the hypervisor
console ring buffer. When enabled, all output to the console
device will be reflected in the ring buffer.  When disabled (the
default), only hypervisor output is available from the ring buffer.

Signed-off-by: Ben Thomas <ben@virtualiron.com>

docs/src/user.tex

@@ -3179,6 +3179,11 @@ editing \path{grub.conf}.
   one of 80x25, 80x28, 80x30, 80x34, 80x43, 80x50, 80x60.
   \item[ keep ] Keep the VGA console even after domain 0 boots.
   \end{description}
+\item [ console\_to\_ring ] Place guest console output into the
+  hypervisor console ring buffer. This is disabled by default.
+  When enabled, both hypervisor output and guest console output
+  is available from the ring buffer. This can be useful for logging
+  and/or remote presentation of console data.
 \item [ sync\_console ] Force synchronous console output. This is
   useful if you system fails unexpectedly before it has sent all
   available output to the console. In most cases Xen will

xen/drivers/char/console.c

@@ -48,6 +48,10 @@ string_param("conswitch", opt_conswitch);
 static int opt_sync_console;
 boolean_param("sync_console", opt_sync_console);
 
+/* console_to_ring: send guest (incl. dom 0) console data to console ring. */
+static int opt_console_to_ring;
+boolean_param("console_to_ring", opt_console_to_ring);
+
 #define CONRING_SIZE 16384
 #define CONRING_IDX_MASK(i) ((i)&(CONRING_SIZE-1))
 static char conring[CONRING_SIZE];
@@ -329,7 +333,14 @@ static long guest_console_write(XEN_GUEST_HANDLE(char) buffer, int count)
         sercon_puts(kbuf);
 
         for ( kptr = kbuf; *kptr != '\0'; kptr++ )
+        {
             vga_putchar(*kptr);
+            if ( opt_console_to_ring )
+                putchar_console_ring(*kptr);
+        }
+
+        if ( opt_console_to_ring )
+            send_guest_global_virq(dom0, VIRQ_CON_RING);
 
         guest_handle_add_offset(buffer, kcount);
         count -= kcount;