Re: [PATCH 2/2] lguest: handle dodgy/non-existent TSC. Guest code.

On Wed, Jul 04, 2007 at 04:20:33PM +1000, Rusty Russell wrote:

We create an "lguest_clock" which the guest uses: either TSC or
jiffies, depending on whether the host tells us the TSC frequency.

Signed-off-by: Rusty Russell <redacted>
---
 drivers/lguest/lguest.c |   51 +++++++++++++++++++++++------------------------
 1 file changed, 26 insertions(+), 25 deletions(-)

===================================================================

Hi Rusty,

<snip>

+static cycle_t lguest_clock_read(void)
+{
+	if (lguest_data.tsc_khz)
+		return native_read_tsc();
+	else
+		return jiffies;
+}
+
+/* This is what we tell the kernel is our clocksource.  */

<snip>

+	/* We use the TSC if the Host tells us we can, otherwise a dumb
+	 * jiffies-based clock. */
+	if (lguest_data.tsc_khz) {
+		lguest_clock.shift = 22;
+		lguest_clock.mult = clocksource_khz2mult(lguest_data.tsc_khz,
+							 lguest_clock.shift);
+		lguest_clock.mask = CLOCKSOURCE_MASK(64);
+		lguest_clock.flags = CLOCK_SOURCE_IS_CONTINUOUS;
+	} else {
+		/* To understand this, start at kernel/time/jiffies.c... */
+		lguest_clock.shift = 8;
+		lguest_clock.mult = (((u64)NSEC_PER_SEC<<8)/ACTHZ) << 8;
+		lguest_clock.mask = CLOCKSOURCE_MASK(32);
+	}
 	clocksource_register(&lguest_clock);

Why bother installing an lguest_clock which is the same as jiffies?
Wouldn't it be better to just use the system provided jiffies
clocksource in the case where you haven't been provided with
lguest_data.tsc_khz ?

Yours Tony

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