linux-kernel - Re: [PATCH] arch/x86/entry_fred: don't set up KVM IRQs if KVM is disabled

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Date: Fri, 16 Feb 2024 22:45:45 +0100
From: Thomas Gleixner <tglx@...utronix.de>
To: Paolo Bonzini <pbonzini@...hat.com>, Xin Li <xin@...or.com>, Sean
 Christopherson <seanjc@...gle.com>, Max Kellermann
 <max.kellermann@...os.com>
Cc: hpa@...or.com, x86@...nel.org, linux-kernel@...r.kernel.org, Stephen
 Rothwell <sfr@...b.auug.org.au>, kvm@...r.kernel.org
Subject: Re: [PATCH] arch/x86/entry_fred: don't set up KVM IRQs if KVM is
 disabled

On Fri, Feb 16 2024 at 07:31, Paolo Bonzini wrote:
> On 2/16/24 03:10, Xin Li wrote:
>
> It is intentional that KVM-related things are compiled out completely
> if !IS_ENABLED(CONFIG_KVM), because then it's also not necessary to
> have

That's a matter of taste. In both cases _ALL_ KVM related things are
compiled out.

#ifdeffing out the vector numbers is silly to begin with because these
vector numbers stay assigned to KVM whether KVM is enabled or not.

And no, I don't think it's a net win to have the #ifdeffery in that
table. Look at apic_idts[] in arch/x86/kernel/idt.c how this ends up
looking. It's unreadable gunk.

The few NULL defines in a header file next to the real stuff

#if IS_ENABLED(CONFIG_KVM)
....
#else
# define fred_sysvec_kvm_posted_intr_ipi                NULL
# define fred_sysvec_kvm_posted_intr_wakeup_ipi         NULL
# define fred_sysvec_kvm_posted_intr_nested_ipi         NULL
#endif

are not hurting at all and they are at a place where #ifdeffery is
required anyway. That's a very common pattern all over the kernel and it
limits the #ifdef horror to _ONE_ place.

With your change you propagate the #ifdefffery to the multiple and the
very wrong places for absolutely zero practical value. The resulting
binary code is exactly the same for the price of tasteless #ifdeffery in
places where it matters.

Please get rid of this #ifdef in the vector header and don't inflict
bad taste on everyone.

Thanks,

        tglx