[PATCH 2/2] lguest: sanitize the clock.

Rusty Russell
2008-03-04T12:12:30Z

Now the TSC code handles a zero return from calculate_cpu_khz(),
lguest can simply pass through the value it gets from the Host: if
non-zero, all the normal TSC code applies.

Otherwise (or if the Host really doesn't support TSC), the clocksource
code will fall back to the slower but reasonable lguest clock.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

diff -r f9a80502dc46 arch/x86/lguest/boot.c
--- a/arch/x86/lguest/boot.c Tue Mar 04 21:06:41 2008 +1100
+++ b/arch/x86/lguest/boot.c Tue Mar 04 22:55:10 2008 +1100
@@ -84,7 +84,6 @@ struct lguest_data lguest_data = {
.blocked_interrupts = { 1 }, /* Block timer interrupts */
.syscall_vec = SYSCALL_VECTOR,
};
-static cycle_t clock_base;

/*G:037 async_hcall() is pretty simple: I'm quite proud of it really. We have a
* ring buffer of stored hypercalls which the Host will run though next time we
@@ -327,8 +326,8 @@ static void lguest_cpuid(unsigned int *a
case 1: /* Basic feature request. */
/* We only allow kernel to see SSE3, CMPXCHG16B and SSSE3 */
*cx &= 0x00002201;
- /* SSE, SSE2, FXSR, MMX, CMOV, CMPXCHG8B, FPU. */
- *dx &= 0x07808101;
+ /* SSE, SSE2, FXSR, MMX, CMOV, CMPXCHG8B, TSC, FPU. */
+ *dx &= 0x07808111;
/* The Host can do a nice optimization if it knows that the
* kernel mappings (addresses above 0xC0000000 or whatever
* PAGE_OFFSET is set to) haven't changed. But Linux calls
@@ -595,19 +594,25 @@ static unsigned long lguest_get_wallcloc
return lguest_data.time.tv_sec;
}

+/* The TSC is a Time Stamp Counter. The Host tells us what speed it runs at,
+ * or 0 if it's unusable as a reliable clock source. This matches what we want
+ * here: if we return 0 from this function, the x86 TSC clock will not register
+ * itself. */
+static unsigned long lguest_cpu_khz(void)
+{
+ return lguest_data.tsc_khz;
+}
+
+/* If we can't use the TSC, the kernel falls back to our "lguest_clock", where
+ * we read the time value given to us by the Host. */
static cycle_t lguest_clock_read(void)
{
unsigned long sec, nsec;

- /* If the Host tells the TSC speed, we can trust that. */
- if (lguest_data.tsc_khz)
- return native_read_tsc();
-
- /* If we can't use the TSC, we read the time value written by the Host.
- * Since it's in two parts (seconds and nanoseconds), we risk reading
- * it just as it's changing from 99 & 0.999999999 to 100 and 0, and
- * getting 99 and 0. As Linux tends to come apart under the stress of
- * time travel, we must be careful: */
+ /* Since the time is in two parts (seconds and nanoseconds), we risk
+ * reading it just as it's changing from 99 & 0.999999999 to 100 and 0,
+ * and getting 99 and 0. As Linux tends to come apart under the stress
+ * of time travel, we must be careful: */
do {
/* First we read the seconds part. */
sec = lguest_data.time.tv_sec;
@@ -622,26 +627,20 @@ static cycle_t lguest_clock_read(void)
/* Now if the seconds part has changed, try again. */
} while (unlikely(lguest_data.time.tv_sec != sec));

- /* Our non-TSC clock is in real nanoseconds. */
+ /* Our lguest clock is in real nanoseconds. */
return sec*1000000000ULL + nsec;
}

-/* This is what we tell the kernel is our clocksource. */
+/* This is the fallback clocksource: lower priority than the TSC clocksource. */
static struct clocksource lguest_clock = {
.name = "lguest",
- .rating = 400,
+ .rating = 200,
.read = lguest_clock_read,
.mask = CLOCKSOURCE_MASK(64),
.mult = 1 << 22,
.shift = 22,
.flags = CLOCK_SOURCE_IS_CONTINUOUS,
};
-
-/* The "scheduler clock" is just our real clock, adjusted to start at zero */
-static unsigned long long lguest_sched_clock(void)
-{
- return cyc2ns(&lguest_clock, lguest_clock_read() - clock_base);
-}

/* We also need a "struct clock_event_device": Linux asks us to set it to go
* off some time in the future. Actually, James Morris figured all this out, I
@@ -712,18 +711,7 @@ static void lguest_time_init(void)
/* Set up the timer interrupt (0) to go to our simple timer routine */
set_irq_handler(0, lguest_time_irq);

- /* Our clock structure looks like arch/x86/kernel/tsc_32.c if we can
- * use the TSC, otherwise it's a dumb nanosecond-resolution clock.
- * Either way, the "rating" is set so high that it's always chosen over
- * any other clocksource. */
- if (lguest_data.tsc_khz)
- lguest_clock.mult = clocksource_khz2mult(lguest_data.tsc_khz,
- lguest_clock.shift);
- clock_base = lguest_clock_read();
clocksource_register(&lguest_clock);
-
- /* Now we've set up our clock, we can use it as the scheduler clock */
- pv_time_ops.sched_clock = lguest_sched_clock;

/* We can't set cpumask in the initializer: damn C limitations! Set it
* here and register our timer device. */
@@ -995,6 +983,7 @@ __init void lguest_init(void)
/* time operations */
pv_time_ops.get_wallclock = lguest_get_wallclock;
pv_time_ops.time_init = lguest_time_init;
+ pv_time_ops.get_cpu_khz = lguest_cpu_khz;

/* Now is a good time to look at the implementations of these functions
* before returning to the rest of lguest_init(). */