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KVM: x86: fix kvm_write_tsc() TSC matching thinko

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kvm_write_tsc() converts from guest TSC to microseconds, not nanoseconds
as intended. The result is that the window for matching is 1000 seconds,
not 1 second.

Microsecond precision is enough for checking whether the TSC write delta
is within the heuristic values, so use it instead of nanoseconds.

Noted by Avi Kivity.

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>
This commit is contained in:
Marcelo Tosatti 2012-03-08 18:46:57 -03:00 committed by Avi Kivity
parent b74f05d61b
commit 02626b6af5
1 changed files with 10 additions and 9 deletions
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19
arch/x86/kvm/x86.c
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@ -1025,7 +1025,7 @@ void kvm_write_tsc(struct kvm_vcpu *vcpu, u64 data)
struct kvm *kvm = vcpu->kvm;
u64 offset, ns, elapsed;
unsigned long flags;
s64 nsdiff;
s64 usdiff;
raw_spin_lock_irqsave(&kvm->arch.tsc_write_lock, flags);
offset = kvm_x86_ops->compute_tsc_offset(vcpu, data);
@ -1033,18 +1033,19 @@ void kvm_write_tsc(struct kvm_vcpu *vcpu, u64 data)
elapsed = ns - kvm->arch.last_tsc_nsec;
/* n.b - signed multiplication and division required */
nsdiff = data - kvm->arch.last_tsc_write;
usdiff = data - kvm->arch.last_tsc_write;
#ifdef CONFIG_X86_64
nsdiff = (nsdiff * 1000) / vcpu->arch.virtual_tsc_khz;
usdiff = (usdiff * 1000) / vcpu->arch.virtual_tsc_khz;
#else
/* do_div() only does unsigned */
asm("idivl %2; xor %%edx, %%edx"
: "=A"(nsdiff)
: "A"(nsdiff * 1000), "rm"(vcpu->arch.virtual_tsc_khz));
: "=A"(usdiff)
: "A"(usdiff * 1000), "rm"(vcpu->arch.virtual_tsc_khz));
#endif
nsdiff -= elapsed;
if (nsdiff < 0)
nsdiff = -nsdiff;
do_div(elapsed, 1000);
usdiff -= elapsed;
if (usdiff < 0)
usdiff = -usdiff;
/*
* Special case: TSC write with a small delta (1 second) of virtual
@ -1056,7 +1057,7 @@ void kvm_write_tsc(struct kvm_vcpu *vcpu, u64 data)
* compensation code attempt to catch up if we fall behind, but
* it's better to try to match offsets from the beginning.
*/
if (nsdiff < NSEC_PER_SEC &&
if (usdiff < USEC_PER_SEC &&
vcpu->arch.virtual_tsc_khz == kvm->arch.last_tsc_khz) {
if (!check_tsc_unstable()) {
offset = kvm->arch.cur_tsc_offset;