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From: Thomas Gleixner <tglx@linutronix.de>
To: Paolo Bonzini <pbonzini@redhat.com>, Xin Li <xin@zytor.com>, Sean
 Christopherson <seanjc@google.com>, Max Kellermann
 <max.kellermann@ionos.com>
Cc: hpa@zytor.com, x86@kernel.org, linux-kernel@vger.kernel.org, Stephen
 Rothwell <sfr@canb.auug.org.au>, kvm@vger.kernel.org
Subject: Re: [PATCH] arch/x86/entry_fred: don't set up KVM IRQs if KVM is
 disabled
In-Reply-To: <5a332064-0a26-4bb9-8a3e-c99604d2d919@redhat.com>
References: <20240215133631.136538-1-max.kellermann@ionos.com>
 <Zc5sMmT20kQmjYiq@google.com>
 <a61b113c-613c-41df-80a5-b061889edfdf@zytor.com>
 <5a332064-0a26-4bb9-8a3e-c99604d2d919@redhat.com>
Date: Fri, 16 Feb 2024 22:45:45 +0100
Message-ID: <87ttm8axrq.ffs@tglx>
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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