[PATCH 10/17] Make KVM clock computation work for other scales

[Zachary Amsden]

The math in kvm_get_time_scale relies on the fact that
NSEC_PER_SEC < 2^32.  To use the same function to compute
arbitrary time scales, we must extend the first reduction
step to shrink the base rate to a 32-bit value, and
possibly reduce the scaled rate into a 32-bit as well.

Note we must take care to avoid an arithmetic overflow
when scaling up the tps32 value (this could not happen
with the fixed scaled value of NSEC_PER_SEC, but can
happen with scaled rates above 2^31.

Signed-off-by: Zachary Amsden <zamsden@xxxxxxxxxx>
---
 arch/x86/kvm/x86.c |   26 +++++++++++++++-----------
 1 files changed, 15 insertions(+), 11 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 05c559d..52d7d34 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -882,31 +882,35 @@ static uint32_t div_frac(uint32_t dividend, uint32_t divisor)
 	return quotient;
 }
 
-static void kvm_set_time_scale(uint32_t tsc_khz, struct pvclock_vcpu_time_info *hv_clock)
+static void kvm_get_time_scale(uint32_t scaled_khz, uint32_t base_khz,
+			       s8 *pshift, u32 *pmultiplier)
 {
-	uint64_t nsecs = 1000000000LL;
+	uint64_t scaled64;
 	int32_t  shift = 0;
 	uint64_t tps64;
 	uint32_t tps32;
 
-	tps64 = tsc_khz * 1000LL;
-	while (tps64 > nsecs*2) {
+	tps64 = base_khz * 1000LL;
+	scaled64 = scaled_khz * 1000LL;
+	while (tps64 > scaled64*2 || tps64 & 0xffffffff00000000UL) {
 		tps64 >>= 1;
 		shift--;
 	}
 
 	tps32 = (uint32_t)tps64;
-	while (tps32 <= (uint32_t)nsecs) {
-		tps32 <<= 1;
+	while (tps32 <= scaled64 || scaled64 & 0xffffffff00000000UL) {
+		if (scaled64 & 0xffffffff00000000UL || tps32 & 0x80000000)
+			scaled64 >>= 1;
+		else
+			tps32 <<= 1;
 		shift++;
 	}
 
-	hv_clock->tsc_shift = shift;
-	hv_clock->tsc_to_system_mul = div_frac(nsecs, tps32);
+	*pshift = shift;
+	*pmultiplier = div_frac(scaled64, tps32);
 
-	pr_debug("%s: tsc_khz %u, tsc_shift %d, tsc_mul %u\n",
-		 __func__, tsc_khz, hv_clock->tsc_shift,
-		 hv_clock->tsc_to_system_mul);
+	pr_debug("%s: base_khz %u => %u, shift %d, mul %u\n",
+		 __func__, base_khz, scaled_khz, shift, *pmultiplier);
 }
 
 static DEFINE_PER_CPU(unsigned long, cpu_tsc_khz);
-- 
1.7.1

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