linux-kernel - Re: [PART1 RFC 5/9] svm: Add VMEXIT handlers for AVIC

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Date:	Fri, 19 Feb 2016 18:32:54 +0700
From:	Suravee Suthikulpanit <Suravee.Suthikulpanit@....com>
To:	Radim Krčmář <rkrcmar@...hat.com>
CC:	Paolo Bonzini <pbonzini@...hat.com>, <joro@...tes.org>,
	<alex.williamson@...hat.com>, <gleb@...nel.org>,
	<kvm@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<wei@...hat.com>, <sherry.hurwitz@....com>
Subject: Re: [PART1 RFC 5/9] svm: Add VMEXIT handlers for AVIC

Hi,

On 2/18/16 21:18, Radim Krčmář wrote:
> 2016-02-18 09:25+0700, Suravee Suthikulpanit:
>> On 2/17/16 01:06, Radim Krčmář wrote:
>>> 2016-02-16 17:56+0100, Paolo Bonzini:
>>>>> On 16/02/2016 15:13, Radim Krčmář wrote:
>>>>>>> Yeah, I think atomic there means that it won't race with other writes to
>>>>>>> the same byte in IRR.  We're fine as long as AVIC writes IRR before
>>>>>>> checking IsRunning on every destination, which it seems to be.
>>>>>
>>>>> More precisely, if AVIC writes all IRRs (5.1) and ANDs all IsRunning
>>>>> flags before checking the result of the AND (6).
>>>>>
>>>>>>> (It would, but I believe that AVIC designers made it sane and the spec
>>>>>>>   doesn't let me read it in a way that supports your theories.)
>>>>>
>>>>> I hope so as well, and you've probably convinced me.  But I still think
>>>>> the code is wrong in this patch.  Let's look at the spec that you pasted:
>>> The code definitely is wrong.  I'll be more specific when disagreeing,
>>> sorry.
>>>
>>
>> Would you please be a bit more specific on what you think I am not doing
>> correctly to handle the #VMEXIT in the case of target not running below.
>>
>> +    case AVIC_INCMP_IPI_ERR_TARGET_NOT_RUN:
>> +        kvm_lapic_reg_write(apic, APIC_ICR2, icrh);
>> +        kvm_lapic_reg_write(apic, APIC_ICR, icrl);
>>
>> This is actually not just writing to the register. Please note that writing
>> to APIC_ICR register would also be calling apic_send_ipi(), which results in
>> injecting interrupts to the target core:
>
> Exactly.  Injecting the interrupt in AVIC_INCMP_IPI_ERR_TARGET_NOT_RUN
> handler is causing the double-injection bug that Paolo described.
>
>> Am I missing something?
>
> Probably that AVIC already wrote to all IRRs (and sent appropriate
> doorbells) before this VMEXIT, so KVM shouldn't repeat it.

Ah, Ok I got it now. Thanks for the detail description. I am still 
waiting to hear back from the hardware designer to confirm the HW 
behavior. Meanwhile, I have tried NOT setting the IRR, and only 
kick_vcpu(). And things seem to work fine. Therefore, I think your 
analysis is likely to be correct.

Thanks again,
Suravee