linux-kernel - Re: [PATCH] kvm: lapic: fix broken vcpu hotplug

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Date:   Mon, 22 Jun 2020 18:47:57 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Igor Mammedov <imammedo@...hat.com>, linux-kernel@...r.kernel.org
Cc:     sean.j.christopherson@...el.com, vkuznets@...hat.com,
        kvm@...r.kernel.org, wanpengli@...cent.com
Subject: Re: [PATCH] kvm: lapic: fix broken vcpu hotplug

On 22/06/20 18:08, Igor Mammedov wrote:
> Guest fails to online hotplugged CPU with error
>   smpboot: do_boot_cpu failed(-1) to wakeup CPU#4
> 
> It's caused by the fact that kvm_apic_set_state(), which used to call
> recalculate_apic_map() unconditionally and pulled hotplugged CPU into
> apic map, is updating map conditionally [1] on state change which doesn't
> happen in this case and apic map update is skipped.
> 
> Note:
> new CPU during kvm_arch_vcpu_create() is not visible to
> kvm_recalculate_apic_map(), so all related update calls endup
> as NOP and only follow up kvm_apic_set_state() used to trigger map
> update that counted in hotplugged CPU.
> Fix issue by forcing unconditional update from kvm_apic_set_state(),
> like it used to be.
> 
> 1)
> Fixes: (4abaffce4d25a KVM: LAPIC: Recalculate apic map in batch)
> Signed-off-by: Igor Mammedov <imammedo@...hat.com>
> ---
> PS:
> it's alternative to full revert of [1], I've posted earlier
> https://www.mail-archive.com/linux-kernel@vger.kernel.org/msg2205600.html
> so fii free to pick up whatever is better by now
> ---
>  arch/x86/kvm/lapic.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index 34a7e0533dad..5696831d4005 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -2556,6 +2556,7 @@ int kvm_apic_set_state(struct kvm_vcpu *vcpu, struct kvm_lapic_state *s)
>  	struct kvm_lapic *apic = vcpu->arch.apic;
>  	int r;
>  
> +	apic->vcpu->kvm->arch.apic_map_dirty = true;
>  	kvm_lapic_set_base(vcpu, vcpu->arch.apic_base);
>  	/* set SPIV separately to get count of SW disabled APICs right */
>  	apic_set_spiv(apic, *((u32 *)(s->regs + APIC_SPIV)));
> 

Queued, but it's better to set apic_map_dirty just before the call to
kvm_recalculate_apic_map, or you can have a variant of the race that you
pointed out.

Paolo