linux-kernel - Re: [PATCH 2/2] KVM: x86: allow BSP to handle INIT IPIs like APs do

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Date:	Mon, 8 Feb 2016 18:53:45 +0100
From:	Jan Kiszka <jan.kiszka@....de>
To:	Bruce Rogers <brogers@...e.com>,
	Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Cc:	namit@...technion.ac.il
Subject: Re: [PATCH 2/2] KVM: x86: allow BSP to handle INIT IPIs like APs do

On 2016-02-08 18:38, Bruce Rogers wrote:
>>>> On 2/8/2016 at 10:27 AM, Bruce Rogers wrote: 
>>>>> On 2/8/2016 at 09:40 AM, Paolo Bonzini <pbonzini@...hat.com> wrote: 
>>
>>>
>>> On 08/02/2016 17:33, Bruce Rogers wrote:
>>>>>>>>
>>>>>>>> KVM_MP_STATE_INIT_RECEIVED is what Intel calls the "wait for SIPI"
>>>>>>>> state.  The BSP never gets a SIPI, it goes straight to 0xFFFFFFF0
>>>>>>>> instead.  Can you explain the problem more in detail?
>>>>>>
>>>>>> I suspect this is about sending INIT-SIPI from another CPU, directed to
>>>>>> the BSP, isn't it? We may have to differentiate between CPU (including
>>>>>> system) reset and that IPI case.
>>>> That is correct. In looking over the KVM code which deals with BSP, this was
>>>> the only place which seemed wrong to me wrt special casing for BSP outside 
>>> the
>>>> context of initial system initialization / reset. As far as I understand the
>>>> BSP shouldn't be treated differently in this case.
>>>
>>> See 8.4.2 of the SDM:
>>>
>>> If the MP protocol has completed and a BSP is chosen, subsequent INITs
>>> (either to a specific processor or system wide) do not cause the MP
>>> protocol to be repeated. Instead, each logical processor examines its
>>> BSP flag (in the IA32_APIC_BASE MSR) to determine whether it should
>>> execute the BIOS boot-strap code (if it is the BSP) or enter a
>>> wait-for-SIPI state (if it is an AP).
>>>
>>> So it is correct to treat the BSP differently here, I think.
>>
>> I had read that, but I though this was speaking from the perspective of the
>> SMP aware BIOS code only. In other words, the BIOS would sidetrack AP's
>> (based on BSP flag not being present), while BSP would be allowed to go 
>> through
>> the regular BIOS code, checking for reset case, etc. An OS on the other hand
>> would be free to treat all x86 processors equally, once it has booted into
>> fully symmetrical mode.
>> I certainly could be wrong about my above interpretation, but with these
>> changes I'm proposing, things work well for the test case of manually 
>> onlining
>> the BSP after the crash kernel has been started (via kexec -e on a AP 
>> processor
>> with maxcpus=1 on the crash kernel command line). From looking through the
>> kernel git history it appears this sequence of events was explicitly 
>> supported
>> quite a while ago, and we've got a customer who uses this for fast recovery 
>> from
>> a guest kernel crash.
>>
>> Bruce
> 
> I mean kexec - p ... above, not kexec -e. Sorry about that.

How does real HW behave with your kexec case? Did you try this?

Jan


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