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

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Date:	Thu, 3 Jul 2014 13:11:42 +0800
From:	Wanpeng Li <wanpeng.li@...ux.intel.com>
To:	Bandan Das <bsd@...hat.com>
Cc:	Paolo Bonzini <pbonzini@...hat.com>,
	Jan Kiszka <jan.kiszka@...mens.com>,
	Gleb Natapov <gleb@...nel.org>,
	Hu Robert <robert.hu@...el.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: nVMX: Fix IRQs inject to L2 which belong to L1
 since race

Hi Bandan,
On Wed, Jul 02, 2014 at 12:27:59PM -0400, Bandan Das wrote:
>Wanpeng Li <wanpeng.li@...ux.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
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