KVM: x86: remaster kvm_write_tsc code

Reuse existing code instead of using inline asm.
Make the code more concise and clear in the TSC
synchronization part.

Signed-off-by: Denis Plotnikov <dplotnikov@virtuozzo.com>
Reviewed-by: Roman Kagan <rkagan@virtuozzo.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>

arch/x86/kvm/x86.c

@@ -1443,10 +1443,10 @@ void kvm_write_tsc(struct kvm_vcpu *vcpu, struct msr_data *msr)
 	struct kvm *kvm = vcpu->kvm;
 	u64 offset, ns, elapsed;
 	unsigned long flags;
-	s64 usdiff;
 	bool matched;
 	bool already_matched;
 	u64 data = msr->data;
+	bool synchronizing = false;
 
 	raw_spin_lock_irqsave(&kvm->arch.tsc_write_lock, flags);
 	offset = kvm_compute_tsc_offset(vcpu, data);
@@ -1454,51 +1454,25 @@ void kvm_write_tsc(struct kvm_vcpu *vcpu, struct msr_data *msr)
 	elapsed = ns - kvm->arch.last_tsc_nsec;
 
 	if (vcpu->arch.virtual_tsc_khz) {
-		int faulted = 0;
-
-		/* n.b - signed multiplication and division required */
-		usdiff = data - kvm->arch.last_tsc_write;
-#ifdef CONFIG_X86_64
-		usdiff = (usdiff * 1000) / vcpu->arch.virtual_tsc_khz;
-#else
-		/* do_div() only does unsigned */
-		asm("1: idivl %[divisor]\n"
-		    "2: xor %%edx, %%edx\n"
-		    "   movl $0, %[faulted]\n"
-		    "3:\n"
-		    ".section .fixup,\"ax\"\n"
-		    "4: movl $1, %[faulted]\n"
-		    "   jmp  3b\n"
-		    ".previous\n"
-
-		_ASM_EXTABLE(1b, 4b)
-
-		: "=A"(usdiff), [faulted] "=r" (faulted)
-		: "A"(usdiff * 1000), [divisor] "rm"(vcpu->arch.virtual_tsc_khz));
-
-#endif
-		do_div(elapsed, 1000);
-		usdiff -= elapsed;
-		if (usdiff < 0)
-			usdiff = -usdiff;
-
-		/* idivl overflow => difference is larger than USEC_PER_SEC */
-		if (faulted)
-			usdiff = USEC_PER_SEC;
-	} else
-		usdiff = USEC_PER_SEC; /* disable TSC match window below */
+		u64 tsc_exp = kvm->arch.last_tsc_write +
+					nsec_to_cycles(vcpu, elapsed);
+		u64 tsc_hz = vcpu->arch.virtual_tsc_khz * 1000LL;
+		/*
+		 * Special case: TSC write with a small delta (1 second)
+		 * of virtual cycle time against real time is
+		 * interpreted as an attempt to synchronize the CPU.
+		 */
+		synchronizing = data < tsc_exp + tsc_hz &&
+				data + tsc_hz > tsc_exp;
+	}
 
 	/*
-	 * Special case: TSC write with a small delta (1 second) of virtual
-	 * cycle time against real time is interpreted as an attempt to
-	 * synchronize the CPU.
-         *
 	 * For a reliable TSC, we can match TSC offsets, and for an unstable
 	 * TSC, we add elapsed time in this computation.  We could let the
 	 * compensation code attempt to catch up if we fall behind, but
 	 * it's better to try to match offsets from the beginning.
          */
-	if (usdiff < USEC_PER_SEC &&
+	if (synchronizing &&
 	    vcpu->arch.virtual_tsc_khz == kvm->arch.last_tsc_khz) {
 		if (!check_tsc_unstable()) {
 			offset = kvm->arch.cur_tsc_offset;