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

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Message-ID: <513DE9F3.9000802@redhat.com>
Date:	Mon, 11 Mar 2013 15:28:03 +0100
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Gleb Natapov <gleb@...hat.com>
CC:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	mtosatti@...hat.com, jan.kiszka@...mens.com
Subject: Re: [PATCH] x86: kvm: reset the bootstrap processor when it gets
 an INIT

Il 11/03/2013 14:54, Gleb Natapov ha scritto:
>> Setting the mp_state to INIT_RECEIVED is that interface, and it already
>> works, for APs at least.  This patch extends it to work for the BSP as well.
>
> It does not for AP either. If AP has vmx on mp_sate should not be set to
> INIT_RECEIVED. mp_sate is a state as you can see from its name and we
> already had a discussion on the generic device API about importance of
> separating sending commands from setting state. There is a difference
> between setting mp_state during migration and signaling INIT#.

What does migration have to do with this?

>> In the corresponding userspace patch, I don't need to touch the CPU
>> state at all.  I can just signal the kernel.  If I touch the CPU, I'll
>> break the nested case, no matter how it is implemented.  So far, the
>> userspace did not have to worry about nested, and that's something that
>> should be kept that way.
> We are discussing two different things here. I'll try to separate them.
> 1. BSP is broken WRT #INIT
> 2. nested is broken WRT #INIT
> 
> You are fixing 1 with your patches, for that I proposed much easier
> solution (at last from kernel point of view): if BSP reset it in
> userspace and make it runnable. Nested virt is still broken, but this is
> not what you are fixing.

It's not what I'm fixing, but I don't want to make the fix for nested
virt unnecessarily more complicated.  Nested virt needs to know about
INIT and SIPI; redefining the meaning of INIT_RECEIVED and SIPI_RECEIVED
makes it more complicated to reflect these events to L1.

> For 2 much more involved fix is needed. Jan fixes it and it will require
> signaling INIT# from userspace by other means than mp_sate because
> signaling INIT# does not automatically means that mp_sate changes to
> INIT_RECEIVED.

In your interpretation of INIT_RECEIVED, no.  In mine, yes...

>> If we move away from the INIT_RECEIVED and SIPI_RECEIVED states for
>> in-kernel APIC -> VCPU communication, then the KVM_SET_MP_STATE ioctl
>> will have to convert them to the right bits in the requests field or in
>> the APIC state.  But I'm starting to see less benefit from moving away
>> from mp_state.
>>
> We are not moving away from mp_state, we are moving away from using
> mp_state for signaling

That's what I meant; sorry for the unclear abbreviation.

> because with nested virt INIT does not always
> change mp_state

Why not?  It does change mp_state, it changes how you react to the
change.  Which is why it's good to have the reset done in kernel space,
not in user space.

Paolo

> , not only that it can change mp_state long after signal
> is received after vmx off is done.
> 
> --
> 			Gleb.
> 

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