linux-kernel - Re: [PATCH 15/19] KVM: x86: Explicitly skip optimized logical map setup if vCPU's LDR==0

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Date:   Wed, 31 Aug 2022 16:41:06 +0300
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,
        Suravee Suthikulpanit <suravee.suthikulpanit@....com>,
        Li RongQing <lirongqing@...du.com>
Subject: Re: [PATCH 15/19] KVM: x86: Explicitly skip optimized logical map
 setup if vCPU's LDR==0

On Wed, 2022-08-31 at 00:35 +0000, Sean Christopherson wrote:
> Explicitly skip the optimized map setup if the vCPU's LDR is '0', i.e. if
> the vCPU will never response to logical mode interrupts.  KVM already
> skips setup in this case, but relies on kvm_apic_map_get_logical_dest()
> to generate mask==0.  KVM still needs the mask=0 check as a non-zero LDR
> can yield mask==0 depending on the mode, but explicitly handling the LDR
> will make it simpler to clean up the logical mode tracking in the future.

If I am not mistaken, the commit description is a bit misleading - in this case we just don't add
the vCPU to the map since it is unreachable, but we do continue creating the map.

Best regards,
	MaxiM Levitsky

> 
> No functional change intended.
> 
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>  arch/x86/kvm/lapic.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index c224b5c7cd92..8209caffe3ab 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -318,10 +318,12 @@ void kvm_recalculate_apic_map(struct kvm *kvm)
>  			continue;
>  
>  		ldr = kvm_lapic_get_reg(apic, APIC_LDR);
> +		if (!ldr)
> +			continue;
>  
>  		if (apic_x2apic_mode(apic)) {
>  			new->mode |= KVM_APIC_MODE_X2APIC;
> -		} else if (ldr) {
> +		} else {
>  			ldr = GET_APIC_LOGICAL_ID(ldr);
>  			if (kvm_lapic_get_reg(apic, APIC_DFR) == APIC_DFR_FLAT)
>  				new->mode |= KVM_APIC_MODE_XAPIC_FLAT;