On Tue, Apr 5, 2016 at 4:28 AM, Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:
>
...
>
> While running my acceptance tests, in one case I got one CPU whose xcr0
> had leaked into the host. This showed up as a SIGILL in strncasecmp's
> AVX code, and a simple program confirmed it:
>
> $ cat xgetbv.c
> #include <stdio.h>
> int main(void)
> {
> unsigned xcr0_h, xcr0_l;
> asm("xgetbv" : "=d"(xcr0_h), "=a"(xcr0_l) : "c"(0));
> printf("%08x:%08x\n", xcr0_h, xcr0_l);
> }
> $ gcc xgetbv.c -O2
> $ for i in `seq 0 55`; do echo $i `taskset -c $i ./a.out`; done|grep -v 007
> 19 00000000:00000003
>
> I'm going to rerun the tests without this patch, as it seems the most
> likely culprit, and leave it out of the pull request if they pass.
Agreed this is a very likely culprit. I think I see one way the
guest's xcr0 can leak into the host. I will do some testing an send
another version. Thanks.
>
> Paolo
>
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index e260ccb..8df1167 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -700,7 +700,6 @@ static int __kvm_set_xcr(struct kvm_vcpu *vcpu, u32 index, u64 xcr)
>> if ((xcr0 & XFEATURE_MASK_AVX512) != XFEATURE_MASK_AVX512)
>> return 1;
>> }
>> - kvm_put_guest_xcr0(vcpu);
>> vcpu->arch.xcr0 = xcr0;
>>
>> if ((xcr0 ^ old_xcr0) & XFEATURE_MASK_EXTEND)
>> @@ -6590,8 +6589,6 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>> kvm_x86_ops->prepare_guest_switch(vcpu);
>> if (vcpu->fpu_active)
>> kvm_load_guest_fpu(vcpu);
>> - kvm_load_guest_xcr0(vcpu);
>> -
>> vcpu->mode = IN_GUEST_MODE;
>>
>> srcu_read_unlock(&vcpu->kvm->srcu, vcpu->srcu_idx);
>> @@ -6607,6 +6604,8 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>>
>> local_irq_disable();
>>
>> + kvm_load_guest_xcr0(vcpu);
>> +
>> if (vcpu->mode == EXITING_GUEST_MODE || vcpu->requests
>> || need_resched() || signal_pending(current)) {
Here, after we've loaded the guest xcr0, if we enter this if
statement, we return from vcpu_enter_guest with the guest's xcr0 still
loaded.
>> vcpu->mode = OUTSIDE_GUEST_MODE;
>> @@ -6667,6 +6666,8 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>> vcpu->mode = OUTSIDE_GUEST_MODE;
>> smp_wmb();
>>
>> + kvm_put_guest_xcr0(vcpu);
>> +
>> /* Interrupt is enabled by handle_external_intr() */
>> kvm_x86_ops->handle_external_intr(vcpu);
>>
>> @@ -7314,7 +7315,6 @@ void kvm_load_guest_fpu(struct kvm_vcpu *vcpu)
>> * and assume host would use all available bits.
>> * Guest xcr0 would be loaded later.
>> */
>> - kvm_put_guest_xcr0(vcpu);
>> vcpu->guest_fpu_loaded = 1;
>> __kernel_fpu_begin();
>> __copy_kernel_to_fpregs(&vcpu->arch.guest_fpu.state);
>> @@ -7323,8 +7323,6 @@ void kvm_load_guest_fpu(struct kvm_vcpu *vcpu)
>>
>> void kvm_put_guest_fpu(struct kvm_vcpu *vcpu)
>> {
>> - kvm_put_guest_xcr0(vcpu);
>> -
>> if (!vcpu->guest_fpu_loaded) {
>> vcpu->fpu_counter = 0;
>> return;
>>
--
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