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

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Date:	Mon, 8 Feb 2016 16:12:23 +0100
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Bruce Rogers <brogers@...e.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Cc:	jan.kiszka@...mens.com, namit@...technion.ac.il
Subject: Re: [PATCH 2/2] KVM: x86: allow BSP to handle INIT IPIs like APs do



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?

Paolo