From 5f4e3f882731c65b5d64a2ff743fda96eaebb9ee Mon Sep 17 00:00:00 2001 From: Zachary Amsden Date: Sat, 18 Sep 2010 14:38:13 -1000 Subject: KVM: x86: Make math work for other scales 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 Signed-off-by: Marcelo Tosatti --- arch/x86/kvm/x86.c | 30 ++++++++++++++++++------------ 1 file changed, 18 insertions(+), 12 deletions(-) (limited to 'arch') diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index 7d2880500fa3..6666af840190 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -920,31 +920,35 @@ 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 inline u64 get_kernel_ns(void) @@ -1084,7 +1088,9 @@ static int kvm_write_guest_time(struct kvm_vcpu *v) } if (unlikely(vcpu->hw_tsc_khz != this_tsc_khz)) { - kvm_set_time_scale(this_tsc_khz, &vcpu->hv_clock); + kvm_get_time_scale(NSEC_PER_SEC / 1000, this_tsc_khz, + &vcpu->hv_clock.tsc_shift, + &vcpu->hv_clock.tsc_to_system_mul); vcpu->hw_tsc_khz = this_tsc_khz; } -- cgit v1.2.3-59-g8ed1b