linux-kernel - Re: [PATCH] x86: kvm: reset the bootstrap processor when it gets an INIT

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Date:	Mon, 11 Mar 2013 18:38:32 +0100
From:	Jan Kiszka <jan.kiszka@...mens.com>
To:	Gleb Natapov <gleb@...hat.com>
CC:	Paolo Bonzini <pbonzini@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"mtosatti@...hat.com" <mtosatti@...hat.com>
Subject: Re: [PATCH] x86: kvm: reset the bootstrap processor when it gets
 an INIT

On 2013-03-11 18:34, Jan Kiszka wrote:
> On 2013-03-11 18:23, Gleb Natapov wrote:
>> On Mon, Mar 11, 2013 at 04:36:33PM +0100, Jan Kiszka wrote:
>>> On 2013-03-11 15:23, Paolo Bonzini wrote:
>>>> Il 11/03/2013 15:05, Gleb Natapov ha scritto:
>>>>> On Mon, Mar 11, 2013 at 03:01:40PM +0100, Jan Kiszka wrote:
>>>>>>> We are not moving away from mp_state, we are moving away from using
>>>>>>> mp_state for signaling because with nested virt INIT does not always
>>>>>>> change mp_state, not only that it can change mp_state long after signal
>>>>>>> is received after vmx off is done.
>>>>>>
>>>>>> Right.
>>>>>>
>>>>>> BTW, for that to happen, we will also need to influence the INIT level.
>>>>>> Unless I misread the spec, INIT is blocked while in root mode, and if
>>>>>> you deassert INIT before leaving root (vmxoff, vmenter), nothing
>>>>>> actually happens. So what matters is the INIT signal level at the exit
>>>>>> of root mode.
>>>>>>
>>>>> You are talking about INIT# signal received via CPU pin, right? I think
>>>>> INIT send by IPI cannot go away.
>>>>
>>>> Neither can go away.  For INIT sent by IPI, 10.4.7 says:
>>>>
>>>> Only the Pentium and P6 family processors support the INIT-deassert IPI.
>>>> An INIT-disassert IPI has no affect on the state of the APIC, other than
>>>> to reload the arbitration ID register with the value in the APIC ID
>>>> register.
>>>>
>>>> 18.27.1 also says that "In the local APIC, NMI and INIT (except for INIT
>>>> deassert) are always treated as edge triggered interrupts".
>>>>
>>>>
>>>> For INIT#, the ICH9 chipset says that "INIT# is driven low for 16 PCI
>>>> clocks" when a soft reset is requested.  So we can guess that INIT# is
>>>> also edge-triggered.
>>>
>>> Ah, ok. So, virtually, INIT stays asserted until it can be delivered in
>>> form of a reset or a vmexit.
>>>
>> vmexit clears it?
> 
> It has to. Otherwise, it would hit the host on vmxoff.
> 
> The spec says: "The INIT signal is blocked whenever a logical processor
> is in VMX root operation. It is not blocked in VMX non-root operation.
> Instead, INITs cause VM exits [...]."

BTW, we have a similar behaviour with SVM, just that GIF and the INIT
interception flags controls the processing: When in guest mode and
interception is enabled, INIT causes vmexit. When in host mode and GIF
is 0, INIT is blocked and held pending until reaching guest mode again
or switching on GIF explicitly.

Jan

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