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

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Date:	Thu, 03 Jul 2014 09:33:52 +0200
From:	Jan Kiszka <jan.kiszka@...mens.com>
To:	Bandan Das <bsd@...hat.com>,
	Wanpeng Li <wanpeng.li@...ux.intel.com>
CC:	Paolo Bonzini <pbonzini@...hat.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

On 2014-07-03 07:29, Bandan Das wrote:
> Wanpeng Li <wanpeng.li@...ux.intel.com> writes:
> 
>> 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.  
> 
> Oh didn't think about that case :), thanks for the pointing this out.
> It's easy to check this with Xen as L1, I suppose.

Xen most probably intercepts external interrupts, but Jailhouse
definitely does not. We also have a unit test for that, but I will
likely not expose the issue of lost events.

Jan

-- 
Siemens AG, Corporate Technology, CT RTC ITP SES-DE
Corporate Competence Center Embedded Linux
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