linux-kernel - Re: [PATCH v4 09/32] KVM: x86: Handle APICv updates for APIC "mode" changes via request

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Message-ID: <fa63b6182c37fec5f9836e039651e0c98c1f9f9f.camel@redhat.com>
Date:   Thu, 08 Dec 2022 23:54:06 +0200
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Alejandro Jimenez <alejandro.j.jimenez@...cle.com>,
        Suravee Suthikulpanit <suravee.suthikulpanit@....com>,
        Li RongQing <lirongqing@...du.com>
Subject: Re: [PATCH v4 09/32] KVM: x86: Handle APICv updates for APIC "mode"
 changes via request

On Sat, 2022-10-01 at 00:58 +0000, Sean Christopherson wrote:
> Use KVM_REQ_UPDATE_APICV to react to APIC "mode" changes, i.e. to handle
> the APIC being hardware enabled/disabled and/or x2APIC being toggled.
> There is no need to immediately update APICv state, the only requirement
> is that APICv be updating prior to the next VM-Enter.
> 
> Making a request will allow piggybacking KVM_REQ_UPDATE_APICV to "inhibit"
> the APICv memslot when x2APIC is enabled.  Doing that directly from
> kvm_lapic_set_base() isn't feasible as KVM's SRCU must not be held when
> modifying memslots (to avoid deadlock), and may or may not be held when
> kvm_lapic_set_base() is called, i.e. KVM can't do the right thing without
> tracking that is rightly buried behind CONFIG_PROVE_RCU=y.
> 
> Suggested-by: Maxim Levitsky <mlevitsk@...hat.com>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>  arch/x86/kvm/lapic.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index 2503f162eb95..316b61b56cca 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -2401,7 +2401,7 @@ void kvm_lapic_set_base(struct kvm_vcpu *vcpu, u64 value)
>  		kvm_apic_set_x2apic_id(apic, vcpu->vcpu_id);
>  
>  	if ((old_value ^ value) & (MSR_IA32_APICBASE_ENABLE | X2APIC_ENABLE)) {
> -		kvm_vcpu_update_apicv(vcpu);
> +		kvm_make_request(KVM_REQ_APICV_UPDATE, vcpu);
>  		static_call_cond(kvm_x86_set_virtual_apic_mode)(vcpu);
>  	}
>  

Makes sense.

Reviewed-by: Maxim Levitsky <mlevitsk@...hat.com>

Best regards,
	Maxim Levitsky