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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 <zamsden@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
This commit is contained in:
Zachary Amsden 2010-09-18 14:38:13 -10:00 committed by Avi Kivity
parent 624d84cfe6
commit 5f4e3f8827
1 changed files with 18 additions and 12 deletions
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28
arch/x86/kvm/x86.c
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@ -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) {
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;
}