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

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Message-Id: <56B86D9B02000048001251EF@prv-mh.provo.novell.com>
Date:	Mon, 08 Feb 2016 10:27:39 -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 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