Commit 5f4e3f88 authored by Zachary Amsden's avatar Zachary Amsden Committed by Avi Kivity
Browse files

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: default avatarZachary Amsden <zamsden@redhat.com>
Signed-off-by: default avatarMarcelo Tosatti <mtosatti@redhat.com>
parent 624d84cf

arch/x86/kvm/x86.c

+18 −12
Original line number Diff line number Diff line
@@ -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;

	}

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