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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>
This commit is contained in:
Marcelo Tosatti 2013-06-11 23:31:12 -03:00 committed by Gleb Natapov
parent 05988d728d
commit 8915aa27d5
1 changed files with 20 additions and 3 deletions
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23
arch/x86/kvm/x86.c
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@ -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 */