KVM: x86: handle idiv overflow at kvm_write_tsc

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Its possible that idivl overflows (due to large delta stored in usdiff,
valid scenario).

Create an exception handler to catch the overflow exception (division by zero
is protected by vcpu->arch.virtual_tsc_khz check), and interpret it accordingly
(delta is larger than USEC_PER_SEC).

Fixes https://bugzilla.redhat.com/show_bug.cgi?id=969644

Signed-off-by: Marcelo Tosatti <mtosatti@xxxxxxxxxx>

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 094b5d9..64a4b03 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1194,20 +1194,37 @@ 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("idivl %2; xor %%edx, %%edx"
-		: "=A"(usdiff)
-		: "A"(usdiff * 1000), "rm"(vcpu->arch.virtual_tsc_khz));
+		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 */
 
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