KVM: x86: handle idiv overflow at kvm_write_tsc
authorMarcelo Tosatti <mtosatti@redhat.com>
Wed, 12 Jun 2013 02:31:12 +0000 (23:31 -0300)
committerGleb Natapov <gleb@redhat.com>
Wed, 12 Jun 2013 11:24:11 +0000 (14:24 +0300)
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>
arch/x86/kvm/x86.c

index 6402951..737c804 100644 (file)
@@ -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 */