KVM: x86: handle idiv overflow at kvm_write_tsc

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@redhat.com>
Signed-off-by: Gleb Natapov <gleb@redhat.com>

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 6402951..737c804 100644 (file)
--- 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 */