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

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Date:	Mon, 11 Mar 2013 12:25:57 +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 11:28, Gleb Natapov ha scritto:
>> Not really true---we do exit with that state and EINTR when we get a
>> SIPI.  Perhaps that can be changed.
>
> That's implementation detail. We can jump to the beginning of the
> function instead. Nowhere we document that entering
> KVM_MP_STATE_SIPI_RECEIVED state cause KVM_RUN return with EINTR.

Yes, that would be nice.

>>> If AP is hard reset
>>> userspase makes it UNINIT, if soft reset it makes it INIT_RECEIVED, if
>>> BSP it makes it running no matter what type of reset.
>>
>> The current name just suggests . 
>> And when getting an INIT in the in-kernel LAPIC, this:
>>
>> -			vcpu->arch.mp_state = KVM_MP_STATE_INIT_RECEIVED;
>> +			vcpu->arch.mp_state = kvm_vcpu_is_bsp(vcpu) ?
>> +				KVM_MP_STATE_SIPI_RECEIVED :
>> +				KVM_MP_STATE_INIT_RECEIVED;
>>
>> makes much less sense than this:
>>
>> -			vcpu->arch.mp_state = KVM_MP_STATE_WAIT_FOR_SIPI;
>> +			vcpu->arch.mp_state = kvm_vcpu_is_bsp(vcpu) ?
>> +				KVM_MP_STATE_RESET_NOW :
>> +				KVM_MP_STATE_WAIT_FOR_SIPI;
>>
> Both of them are equally incorrect. INIT should cause reset, and only if
> vmx is off. An userspace reset is also completely broken in that regard.
> Renaming things gives us nothing, only bring unneeded churn. If the
> names were internal I wouldn't mind, but they are APIs.
> 
>> However, there's also Jan's plans for nVMX.  Peeking at his queue (see
>> http://git.kiszka.org/?p=linux-kvm.git;a=commitdiff;h=037fb24ec) I think
>> it's better to always reflect INITs to the hypervisor like I did in these
>> patches.
>>
> The commit was before we decided that we should not abuse mp_state for
> signaling.

Agreed, but we still have the problem of how to signal from userspace.
For that do you have any other suggestion than mp_state?  And if we keep
mp_state to signal from userspace, giving INIT_RECEIVED the
"wait-for-SIPI" semantics would be wrong.

Paolo

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