linux-kernel - Re: [PATCH v2 3/3] KVM: X86: Fixup kvm_apic_match_dest() dest

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Date:   Mon, 02 Dec 2019 10:18:00 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Peter Xu <peterx@...hat.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     Nitesh Narayan Lal <nitesh@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        peterx@...hat.com
Subject: Re: [PATCH v2 3/3] KVM: X86: Fixup kvm_apic_match_dest() dest_mode parameter

Peter Xu <peterx@...hat.com> writes:

> The problem is the same as the previous patch on that we've got too
> many ways to define a dest_mode, but logically we should only pass in
> APIC_DEST_* macros for this helper.

Using 'the previous patch' in changelog is OK until it comes to
backporting as the order can change. I'd suggest to spell out "KVM: X86:
Use APIC_DEST_* macros properly" explicitly.

>
> To make it easier, simply define dest_mode of kvm_apic_match_dest() to
> be a boolean to make it right while we can avoid to touch the callers.
>
> Signed-off-by: Peter Xu <peterx@...hat.com>
> ---
>  arch/x86/kvm/lapic.c | 5 +++--
>  arch/x86/kvm/lapic.h | 2 +-
>  2 files changed, 4 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index cf9177b4a07f..80732892c709 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -791,8 +791,9 @@ static u32 kvm_apic_mda(struct kvm_vcpu *vcpu, unsigned int dest_id,
>  	return dest_id;
>  }
>  
> +/* Set dest_mode to true for logical mode, false for physical mode */
>  bool kvm_apic_match_dest(struct kvm_vcpu *vcpu, struct kvm_lapic *source,
> -			   int short_hand, unsigned int dest, int dest_mode)
> +			   int short_hand, unsigned int dest, bool dest_mode)
>  {
>  	struct kvm_lapic *target = vcpu->arch.apic;
>  	u32 mda = kvm_apic_mda(vcpu, dest, source, target);
> @@ -800,7 +801,7 @@ bool kvm_apic_match_dest(struct kvm_vcpu *vcpu, struct kvm_lapic *source,
>  	ASSERT(target);
>  	switch (short_hand) {
>  	case APIC_DEST_NOSHORT:
> -		if (dest_mode == APIC_DEST_PHYSICAL)
> +		if (dest_mode == false)

I must admit this seriously harm the readability of the code for
me. Just look at the 

 if (dest_mode == false)

line without a context and try to say what's being checked. I can't.

I see to solutions:
1) Adhere to the APIC_DEST_PHYSICAL/APIC_DEST_LOGICAL (basically - just
check against "dest_mode == APIC_DEST_LOGICAL" in the else branch)
2) Rename the dest_mode parameter to 'dest_mode_is_phys' or something
like that.

>  			return kvm_apic_match_physical_addr(target, mda);
>  		else
>  			return kvm_apic_match_logical_addr(target, mda);
> diff --git a/arch/x86/kvm/lapic.h b/arch/x86/kvm/lapic.h
> index 19b36196e2ff..c0b472ed87ad 100644
> --- a/arch/x86/kvm/lapic.h
> +++ b/arch/x86/kvm/lapic.h
> @@ -82,7 +82,7 @@ int kvm_lapic_reg_write(struct kvm_lapic *apic, u32 reg, u32 val);
>  int kvm_lapic_reg_read(struct kvm_lapic *apic, u32 offset, int len,
>  		       void *data);
>  bool kvm_apic_match_dest(struct kvm_vcpu *vcpu, struct kvm_lapic *source,
> -			   int short_hand, unsigned int dest, int dest_mode);
> +			   int short_hand, unsigned int dest, bool dest_mode);
>  int kvm_apic_compare_prio(struct kvm_vcpu *vcpu1, struct kvm_vcpu *vcpu2);
>  int kvm_irq_delivery_to_apic(struct kvm *kvm, struct kvm_lapic *src,
>  			     struct kvm_lapic_irq *irq,

-- 
Vitaly