linux-kernel - Re: [PATCH v2 9/9] KVM: x86: Short-circuit all of kvm_apic_set

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Message-ID: <004816c9b56ac5b9a56592578caa3a5941045788.camel@intel.com>
Date: Mon, 4 Nov 2024 10:59:02 +0000
From: "Huang, Kai" <kai.huang@...el.com>
To: "pbonzini@...hat.com" <pbonzini@...hat.com>, "seanjc@...gle.com"
	<seanjc@...gle.com>
CC: "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 9/9] KVM: x86: Short-circuit all of kvm_apic_set_base()
 if MSR value is unchanged

On Fri, 2024-11-01 at 11:35 -0700, Sean Christopherson wrote:
> Do nothing in from kvm_apic_set_base() if the incoming MSR value is the
> same as the current value, as validating the mode transitions is obviously
> unnecessary, and rejecting the write is pointless if the vCPU already has
> an invalid value, e.g. if userspace is doing weird things and modified
> guest CPUID after setting MSR_IA32_APICBASE.
> 
> Bailing early avoids kvm_recalculate_apic_map()'s slow path in the rare
> scenario where the map is DIRTY due to some other vCPU dirtying the map,
> in which case it's the other vCPU/task's responsibility to recalculate the
> map.
> 
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>  arch/x86/kvm/lapic.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index 7b2342e40e4e..59a64b703aad 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -2582,9 +2582,6 @@ static void __kvm_apic_set_base(struct kvm_vcpu *vcpu, u64 value)
>  	u64 old_value = vcpu->arch.apic_base;
>  	struct kvm_lapic *apic = vcpu->arch.apic;
>  
> -	if (old_value == value)
> -		return;
> -

Could you clarify why this is removed?  AFAICT kvm_lapic_reset() still calls
directly.

>  	vcpu->arch.apic_base = value;
>  
>  	if ((old_value ^ value) & MSR_IA32_APICBASE_ENABLE)
> @@ -2632,6 +2629,10 @@ int kvm_apic_set_base(struct kvm_vcpu *vcpu, u64 value, bool host_initiated)
>  {
>  	enum lapic_mode old_mode = kvm_get_apic_mode(vcpu);
>  	enum lapic_mode new_mode = kvm_apic_mode(value);
> +
> +	if (vcpu->arch.apic_base == value)
> +		return 0;
> +
>  	u64 reserved_bits = kvm_vcpu_reserved_gpa_bits_raw(vcpu) | 0x2ff |
>  		(guest_cpuid_has(vcpu, X86_FEATURE_X2APIC) ? 0 : X2APIC_ENABLE);
>  
> -- 
> 2.47.0.163.g1226f6d8fa-goog
>