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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>
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1 changed files with 18 additions and 12 deletions
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@ -920,31 +920,35 @@ static uint32_t div_frac(uint32_t dividend, uint32_t divisor)
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return quotient;
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}
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static void kvm_set_time_scale(uint32_t tsc_khz, struct pvclock_vcpu_time_info *hv_clock)
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static void kvm_get_time_scale(uint32_t scaled_khz, uint32_t base_khz,
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s8 *pshift, u32 *pmultiplier)
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{
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uint64_t nsecs = 1000000000LL;
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uint64_t scaled64;
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int32_t shift = 0;
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uint64_t tps64;
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uint32_t tps32;
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tps64 = tsc_khz * 1000LL;
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while (tps64 > nsecs*2) {
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tps64 = base_khz * 1000LL;
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scaled64 = scaled_khz * 1000LL;
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while (tps64 > scaled64*2 || tps64 & 0xffffffff00000000UL) {
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tps64 >>= 1;
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shift--;
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}
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tps32 = (uint32_t)tps64;
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while (tps32 <= (uint32_t)nsecs) {
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tps32 <<= 1;
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while (tps32 <= scaled64 || scaled64 & 0xffffffff00000000UL) {
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if (scaled64 & 0xffffffff00000000UL || tps32 & 0x80000000)
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scaled64 >>= 1;
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else
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tps32 <<= 1;
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shift++;
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}
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hv_clock->tsc_shift = shift;
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hv_clock->tsc_to_system_mul = div_frac(nsecs, tps32);
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*pshift = shift;
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*pmultiplier = div_frac(scaled64, tps32);
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pr_debug("%s: tsc_khz %u, tsc_shift %d, tsc_mul %u\n",
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__func__, tsc_khz, hv_clock->tsc_shift,
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hv_clock->tsc_to_system_mul);
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pr_debug("%s: base_khz %u => %u, shift %d, mul %u\n",
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__func__, base_khz, scaled_khz, shift, *pmultiplier);
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}
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static inline u64 get_kernel_ns(void)
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@ -1084,7 +1088,9 @@ static int kvm_write_guest_time(struct kvm_vcpu *v)
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}
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if (unlikely(vcpu->hw_tsc_khz != this_tsc_khz)) {
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kvm_set_time_scale(this_tsc_khz, &vcpu->hv_clock);
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kvm_get_time_scale(NSEC_PER_SEC / 1000, this_tsc_khz,
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&vcpu->hv_clock.tsc_shift,
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&vcpu->hv_clock.tsc_to_system_mul);
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vcpu->hw_tsc_khz = this_tsc_khz;
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}
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