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

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Message-Id: <56B8703502000048001251F9@prv-mh.provo.novell.com>
Date:	Mon, 08 Feb 2016 10:38:45 -0700
From:	"Bruce Rogers" <brogers@...e.com>
To:	"Paolo Bonzini" <pbonzini@...hat.com>, <kvm@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, "Jan Kiszka" <jan.kiszka@....de>
Cc:	<namit@...technion.ac.il>
Subject: Re: [PATCH 2/2] KVM: x86: allow BSP to handle INIT IPIs like
 APs do

>>> 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.

Bruce