KVM: x86: fix kvm_write_tsc() TSC matching thinko

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>

1 files changed, 10 insertions, 9 deletions

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 32096cf6c6c9..7287812eeb72 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -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;