Re: [PATCH] KVM: nVMX: Fix IRQs inject to L2 which belong to L1 since race

Wanpeng Li
2014-07-03T05:10:48Z

Hi Bandan,
On Wed, Jul 02, 2014 at 12:27:59PM -0400, Bandan Das wrote:
>Wanpeng Li <wanpeng.li@linux.intel.com> writes:
>
>> This patch fix bug https://bugzilla.kernel.org/show_bug.cgi?id=72381
>I can also reproduce this easily with Linux as L1 by "slowing it down"
>eg. running with ept = 0
>
>I suggest changing the subject to -
>KVM: nVMX: Fix race that incorrectly injects L1's irq to L2
>

Ok, I will fold this to next version. ;-)

>> If we didn't inject a still-pending event to L1 since nested_run_pending,
>> KVM_REQ_EVENT should be requested after the vmexit in order to inject the
>> event to L1. However, current log blindly request a KVM_REQ_EVENT even if
>
>What's current "log" ? Do you mean current "code" ?
>

Yeah, it's a typo. I mean "logic".

[...]
>Also, I am wondering isn't it enough to just do this to avoid this race ?
>
> static int vmx_interrupt_allowed(struct kvm_vcpu *vcpu)
> {
>- return (!to_vmx(vcpu)->nested.nested_run_pending &&
>+ return (!is_guest_mode(vcpu) &&
>+ !to_vmx(vcpu)->nested.nested_run_pending &&
> vmcs_readl(GUEST_RFLAGS) & X86_EFLAGS_IF) &&
> !(vmcs_read32(GUEST_INTERRUPTIBILITY_INFO) &
>

I don't think you fix the root cause of the race, and there are two cases which
I concern about your proposal:

- If there is a special L1 which don't ask to exit on external intrs, you will
lose the intrs which L0 inject to L2.

- If inject_pending_event fail to inject an intr since the change that you made
in vmx_interrupt_allowed, L0 will request an intr window for L2, however, the
intr is belong to L1.

Regards,
Wanpeng Li

>Thanks,
>Bandan