console: allow log level threshold adjustments

... from serial console so that one doesn't always need to reboot to
see more / less messages.

Note that upper thresholds are sticky, i.e. while they get adjusted
upwards when the lower threshold would otherwise end up above the upper
one, they don't get adjusted when reducing the lower one. Full
flexibility is available only via a future sysctl interface.

Note further that (meaningless) large threshold values aren't being
rejected, for the sake of not adding more checks to the code than are
really necessary for safe operation.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
v4: Split out sysctl.
v3: Mention apparently missing threshold upper bound checks in
    description. Make flask_sysctl() accept the new operation.
v2: Add sysctl.

--- a/xen/drivers/char/console.c
+++ b/xen/drivers/char/console.c
@@ -175,7 +175,7 @@ static void __init parse_guest_loglvl(ch
     _parse_loglvl(s, &xenlog_guest_lower_thresh, &xenlog_guest_upper_thresh);
 }
 
-static char * __init loglvl_str(int lvl)
+static char *loglvl_str(int lvl)
 {
     switch ( lvl )
     {
@@ -188,6 +188,50 @@ static char * __init loglvl_str(int lvl)
     return "???";
 }
 
+static int *__read_mostly upper_thresh_adj = &xenlog_upper_thresh;
+static int *__read_mostly lower_thresh_adj = &xenlog_lower_thresh;
+static const char *__read_mostly thresh_adj = "standard";
+
+static void do_toggle_guest(unsigned char key, struct cpu_user_regs *regs)
+{
+    if ( upper_thresh_adj == &xenlog_upper_thresh )
+    {
+        upper_thresh_adj = &xenlog_guest_upper_thresh;
+        lower_thresh_adj = &xenlog_guest_lower_thresh;
+        thresh_adj = "guest";
+    }
+    else
+    {
+        upper_thresh_adj = &xenlog_upper_thresh;
+        lower_thresh_adj = &xenlog_lower_thresh;
+        thresh_adj = "standard";
+    }
+    printk("'%c' pressed -> %s log level adjustments enabled\n",
+           key, thresh_adj);
+}
+
+static void do_adj_thresh(unsigned char key)
+{
+    if ( *upper_thresh_adj < *lower_thresh_adj )
+        *upper_thresh_adj = *lower_thresh_adj;
+    printk("'%c' pressed -> %s log level: %s (rate limited %s)\n",
+           key, thresh_adj, loglvl_str(*lower_thresh_adj),
+           loglvl_str(*upper_thresh_adj));
+}
+
+static void do_inc_thresh(unsigned char key, struct cpu_user_regs *regs)
+{
+    ++*lower_thresh_adj;
+    do_adj_thresh(key);
+}
+
+static void do_dec_thresh(unsigned char key, struct cpu_user_regs *regs)
+{
+    if ( *lower_thresh_adj )
+        --*lower_thresh_adj;
+    do_adj_thresh(key);
+}
+
 /*
  * ********************************************************
  * *************** ACCESS TO CONSOLE RING *****************
@@ -816,6 +860,12 @@ void __init console_endboot(void)
 
     register_keyhandler('w', dump_console_ring_key,
                         "synchronously dump console ring buffer (dmesg)", 0);
+    register_irq_keyhandler('+', &do_inc_thresh,
+                            "increase log level threshold", 0);
+    register_irq_keyhandler('-', &do_dec_thresh,
+                            "decrease log level threshold", 0);
+    register_irq_keyhandler('G', &do_toggle_guest,
+                            "toggle host/guest log level adjustment", 0);
 
     /* Serial input is directed to DOM0 by default. */
     switch_serial_input();