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

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Message-ID: <56B8B29C.8060600@web.de>
Date:	Mon, 8 Feb 2016 16:22:04 +0100
From:	Jan Kiszka <jan.kiszka@....de>
To:	Paolo Bonzini <pbonzini@...hat.com>,
	Bruce Rogers <brogers@...e.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Cc:	namit@...technion.ac.il
Subject: Re: [PATCH 2/2] KVM: x86: allow BSP to handle INIT IPIs like APs do

On 2016-02-08 16:12, Paolo Bonzini wrote:
> 
> 
> On 03/02/2016 23:51, Bruce Rogers wrote:
>> The INIT IPI event handler special cases the boot-strap processor
>> (BSP) handling, avoiding the same mp state handling which is done for
>> the other (AP) processors. Debugging a linux guest usage scenario of
>> avoiding a reboot through the bios for a crash on any processor via eg:
>> kexec -p /boot/vmlinuz --initrd=/boot/initrd --append="$(cat /proc/cmdline)\
>> maxcpus=1" led to identifying this change as the needed fix.
>>
>> With this change, an AP can now startup the BSP without error.
>>
>> Signed-off-by: Bruce Rogers <brogers@...e.com>
>> ---
>>  arch/x86/kvm/lapic.c | 5 +----
>>  1 file changed, 1 insertion(+), 4 deletions(-)
>>
>> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
>> index 36591fa..eda6bfb 100644
>> --- a/arch/x86/kvm/lapic.c
>> +++ b/arch/x86/kvm/lapic.c
>> @@ -2170,10 +2170,7 @@ void kvm_apic_accept_events(struct kvm_vcpu *vcpu)
>>  	if (test_bit(KVM_APIC_INIT, &pe)) {
>>  		kvm_lapic_reset(vcpu, true);
>>  		kvm_vcpu_reset(vcpu, true);
>> -		if (kvm_vcpu_is_bsp(apic->vcpu))
>> -			vcpu->arch.mp_state = KVM_MP_STATE_RUNNABLE;
>> -		else
>> -			vcpu->arch.mp_state = KVM_MP_STATE_INIT_RECEIVED;
>> +		vcpu->arch.mp_state = KVM_MP_STATE_INIT_RECEIVED;
>>  	}
>>  	if (test_bit(KVM_APIC_SIPI, &pe) &&
>>  	    vcpu->arch.mp_state == KVM_MP_STATE_INIT_RECEIVED) {
>>
> 
> 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.

Jan


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