Re: pv clock: kvm is incompatible with xen :-(

[Gerd Hoffmann <kraxel@redhat.com>]

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Gerd Hoffmann wrote:

> Wall clock is off a few hours though.  Oops.
> 
> I think the way wall clock and system clock work together in xen (Jeremy
> correct me if I'm wrong) is that the wall clock specifies the point in
> time where the system clock started going.  As kvm fills in host system
> time into the guest system time fields the guest wall clock fields
> should be filled with the host boot time timestamp I'd say.

Following up myself with a quick&dirty patch to tackle this issue too.
This one calculates the boot time.  That should be solveable better,
include/linux/time.h lists two functions which sound promising:

  extern void getboottime(struct timespec *ts);
  extern void monotonic_to_bootbased(struct timespec *ts);

Neither of them is available to modules though, so I can't test without
rebooting my laptop ...

monotonic_to_bootbased() sounds like we would get hosts ntp adjustments
in the guests for free.

cheers,
  Gerd

-- 
http://kraxel.fedorapeople.org/xenner/

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diff -up kvm-65/kernel/x86.c.fix kvm-65/kernel/x86.c
--- kvm-65/kernel/x86.c.fix	2008-04-06 21:23:07.000000000 +0200
+++ kvm-65/kernel/x86.c	2008-04-11 16:17:23.000000000 +0200
@@ -490,7 +490,7 @@ static void kvm_write_wall_clock(struct 
 {
 	static int version;
 	struct kvm_wall_clock wc;
-	struct timespec wc_ts;
+	struct timespec now,sys,boot;
 
 	if (!wall_clock)
 		return;
@@ -499,9 +499,11 @@ static void kvm_write_wall_clock(struct 
 
 	kvm_write_guest(kvm, wall_clock, &version, sizeof(version));
 
-	wc_ts = current_kernel_time();
-	wc.wc_sec = wc_ts.tv_sec;
-	wc.wc_nsec = wc_ts.tv_nsec;
+	now = current_kernel_time();
+	ktime_get_ts(&sys);
+	boot = ns_to_timespec(timespec_to_ns(&now) - timespec_to_ns(&sys));
+	wc.wc_sec = boot.tv_sec;
+	wc.wc_nsec = boot.tv_nsec;
 	wc.wc_version = version;
 
 	kvm_write_guest(kvm, wall_clock, &wc, sizeof(wc));
@@ -548,6 +550,44 @@ static void kvm_write_guest_time(struct 
 	mark_page_dirty(v->kvm, vcpu->time >> PAGE_SHIFT);
 }
 
+static uint32_t div_frac(uint32_t dividend, uint32_t divisor)
+{
+	uint32_t quotient, remainder;
+	
+	__asm__ ( "divl %4"
+		  : "=a" (quotient), "=d" (remainder)
+		  : "0" (0), "1" (dividend), "r" (divisor) );
+	return quotient;
+}
+
+static void kvm_set_time_scale(uint32_t tsc_khz, struct kvm_vcpu_time_info *hv_clock)
+{
+	uint64_t nsecs = 1000000000LL;
+	int32_t  shift = 0;
+	uint64_t tps64;
+	uint32_t tps32;
+	
+	tps64 = tsc_khz * 1000LL;
+	while (tps64 > nsecs*2) {
+		tps64 >>= 1;
+		shift--;
+	}
+
+	tps32 = (uint32_t)tps64;
+	while (tps32 <= (uint32_t)nsecs) {
+		tps32 <<= 1;
+		shift++;
+	}
+
+	hv_clock->tsc_shift = shift;
+	hv_clock->tsc_to_system_mul = div_frac(nsecs, tps32);
+
+#if 0
+	printk(KERN_DEBUG "%s: tsc_khz %u, tsc_shift %d, tsc_mul %u\n",
+	       __FUNCTION__, tsc_khz, hv_clock->tsc_shift,
+	       hv_clock->tsc_to_system_mul);
+#endif
+}
 
 int kvm_set_msr_common(struct kvm_vcpu *vcpu, u32 msr, u64 data)
 {
@@ -596,9 +636,7 @@ int kvm_set_msr_common(struct kvm_vcpu *
 		/* ...but clean it before doing the actual write */
 		vcpu->arch.time_offset = data & ~(PAGE_MASK | 1);
 
-		vcpu->arch.hv_clock.tsc_to_system_mul =
-					clocksource_khz2mult(kvm_tsc_khz, 22);
-		vcpu->arch.hv_clock.tsc_shift = 22;
+		kvm_set_time_scale(kvm_tsc_khz, &vcpu->arch.hv_clock);
 
 		down_read(&current->mm->mmap_sem);
 		vcpu->arch.time_page =

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