[HVM] Rate limit guest accesses to the qemu virtual serial port. This stops

grub's boot menu from hammering dom0.

Signed-off-by: Steven Smith <sos22@cam.ac.uk>

diff --git a/tools/ioemu/hw/serial.c b/tools/ioemu/hw/serial.c
index f36beb209fe7051e9b307392dbbdc1b2c220f36e..1db3e5211fb95169d47bc2e707438789c3c7fd88 100644 (file)
--- a/tools/ioemu/hw/serial.c
+++ b/tools/ioemu/hw/serial.c
@@ -22,6 +22,9 @@
  * THE SOFTWARE.
  */
 #include "vl.h"
+#include <sys/time.h>
+#include <time.h>
+#include <assert.h>
 
 //#define DEBUG_SERIAL
 
@@ -140,6 +143,67 @@ static void serial_update_parameters(SerialState *s)
 #endif
 }
 
+/* Rate limit serial requests so that e.g. grub on a serial console
+   doesn't kill dom0.  Simple token bucket.  If we get some actual
+   data from the user, instantly refil the bucket. */
+
+/* How long it takes to generate a token, in microseconds. */
+#define TOKEN_PERIOD 1000
+/* Maximum and initial size of token bucket */
+#define TOKENS_MAX 100000
+
+static int tokens_avail;
+
+static void serial_get_token(void)
+{
+    static struct timeval last_refil_time;
+    static int started;
+
+    assert(tokens_avail >= 0);
+    if (!tokens_avail) {
+       struct timeval delta, now;
+       int generated;
+
+       if (!started) {
+           gettimeofday(&last_refil_time, NULL);
+           tokens_avail = TOKENS_MAX;
+           started = 1;
+           return;
+       }
+    retry:
+       gettimeofday(&now, NULL);
+       delta.tv_sec = now.tv_sec - last_refil_time.tv_sec;
+       delta.tv_usec = now.tv_usec - last_refil_time.tv_usec;
+       if (delta.tv_usec < 0) {
+           delta.tv_usec += 1000000;
+           delta.tv_sec--;
+       }
+       assert(delta.tv_usec >= 0 && delta.tv_sec >= 0);
+       if (delta.tv_usec < TOKEN_PERIOD) {
+           struct timespec ts;
+           /* Wait until at least one token is available. */
+           ts.tv_sec = TOKEN_PERIOD / 1000000;
+           ts.tv_nsec = (TOKEN_PERIOD % 1000000) * 1000;
+           while (nanosleep(&ts, &ts) < 0 && errno == EINTR)
+               ;
+           goto retry;
+       }
+       generated = (delta.tv_sec * 1000000) / TOKEN_PERIOD;
+       generated +=
+           ((delta.tv_sec * 1000000) % TOKEN_PERIOD + delta.tv_usec) / TOKEN_PERIOD;
+       assert(generated > 0);
+
+       last_refil_time.tv_usec += (generated * TOKEN_PERIOD) % 1000000;
+       last_refil_time.tv_sec  += last_refil_time.tv_usec / 1000000;
+       last_refil_time.tv_usec %= 1000000;
+       last_refil_time.tv_sec  += (generated * TOKEN_PERIOD) / 1000000;
+       if (generated > TOKENS_MAX)
+           generated = TOKENS_MAX;
+       tokens_avail = generated;
+    }
+    tokens_avail--;
+}
+
 static void serial_ioport_write(void *opaque, uint32_t addr, uint32_t val)
 {
     SerialState *s = opaque;
@@ -245,9 +309,11 @@ static uint32_t serial_ioport_read(void *opaque, uint32_t addr)
         ret = s->mcr;
         break;
     case 5:
+       serial_get_token();
         ret = s->lsr;
         break;
     case 6:
+       serial_get_token();
         if (s->mcr & UART_MCR_LOOP) {
             /* in loopback, the modem output pins are connected to the
                inputs */
@@ -296,12 +362,14 @@ static int serial_can_receive1(void *opaque)
 static void serial_receive1(void *opaque, const uint8_t *buf, int size)
 {
     SerialState *s = opaque;
+    tokens_avail = TOKENS_MAX;
     serial_receive_byte(s, buf[0]);
 }
 
 static void serial_event(void *opaque, int event)
 {
     SerialState *s = opaque;
+    tokens_avail = TOKENS_MAX;
     if (event == CHR_EVENT_BREAK)
         serial_receive_break(s);
 }