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 09:41:45 -0800
From: Xin Li <xin@...or.com>
To: Paolo Bonzini <pbonzini@...hat.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 2/15/2024 10:31 PM, Paolo Bonzini wrote:
> On 2/16/24 03:10, Xin Li wrote:
>> On 2/15/2024 11:55 AM, Sean Christopherson wrote:
>>> +Paolo and Stephen
>>>
>>> FYI, there's a build failure in -next due to a collision between 
>>> kvm/next and
>>> tip/x86/fred.  The above makes everything happy.
>>>
>>> On Thu, Feb 15, 2024, Max Kellermann wrote:
>>>> When KVM is disabled, the POSTED_INTR_* macros do not exist, and the
>>>> build fails.
>>>>
>>>> Fixes: 14619d912b65 ("x86/fred: FRED entry/exit and dispatch code")
>>>> Signed-off-by: Max Kellermann <max.kellermann@...os.com>
>>>> ---
>>>>   arch/x86/entry/entry_fred.c | 2 ++
>>>>   1 file changed, 2 insertions(+)
>>>>
>>>> diff --git a/arch/x86/entry/entry_fred.c b/arch/x86/entry/entry_fred.c
>>>> index ac120cbdaaf2..660b7f7f9a79 100644
>>>> --- a/arch/x86/entry/entry_fred.c
>>>> +++ b/arch/x86/entry/entry_fred.c
>>>> @@ -114,9 +114,11 @@ static idtentry_t 
>>>> sysvec_table[NR_SYSTEM_VECTORS] __ro_after_init = {
>>>>       SYSVEC(IRQ_WORK_VECTOR,            irq_work),
>>>> +#if IS_ENABLED(CONFIG_KVM)
>>>>       SYSVEC(POSTED_INTR_VECTOR,        kvm_posted_intr_ipi),
>>>>       SYSVEC(POSTED_INTR_WAKEUP_VECTOR,    kvm_posted_intr_wakeup_ipi),
>>>>       SYSVEC(POSTED_INTR_NESTED_VECTOR,    kvm_posted_intr_nested_ipi),
>>>> +#endif
>>>>   };
>>>>   static bool fred_setup_done __initdata;
>>>> -- 
>>>> 2.39.2
>>
>> We want to minimize #ifdeffery (which is why we didn't add any to
>> sysvec_table[]), would it be better to simply remove "#if 
>> IS_ENABLED(CONFIG_KVM)" around the the POSTED_INTR_* macros from the
>> Linux-next tree?
>>
>> BTW, kvm_posted_intr_*() are defined to NULL if !IS_ENABLED(CONFIG_KVM).
> 
> It is intentional that KVM-related things are compiled out completely
> if !IS_ENABLED(CONFIG_KVM), 

In arch/x86/include/asm/irq_vectors.h, most vector definitions are not
under any #ifdeffery, e.g., THERMAL_APIC_VECTOR not under
CONFIG_X86_THERMAL_VECTOR and IRQ_WORK_VECTOR not under CONFIG_IRQ_WORK.

We'd better make all of them consistent, and the question is that should
we add #ifdefs or not.

> because then it's also not necessary to have
> 
> # 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
> 
> in arch/x86/include/asm/idtentry.h. The full conflict resultion is
> 
> diff --git a/arch/x86/entry/entry_fred.c b/arch/x86/entry/entry_fred.c
> index ac120cbdaaf2..660b7f7f9a79 100644
> --- a/arch/x86/entry/entry_fred.c
> +++ b/arch/x86/entry/entry_fred.c
> @@ -114,9 +114,11 @@ static idtentry_t sysvec_table[NR_SYSTEM_VECTORS] 
> __ro_after_init = {
> 
>       SYSVEC(IRQ_WORK_VECTOR,            irq_work),
> 
> +#if IS_ENABLED(CONFIG_KVM)
>       SYSVEC(POSTED_INTR_VECTOR,        kvm_posted_intr_ipi),
>       SYSVEC(POSTED_INTR_WAKEUP_VECTOR,    kvm_posted_intr_wakeup_ipi),
>       SYSVEC(POSTED_INTR_NESTED_VECTOR,    kvm_posted_intr_nested_ipi),
> +#endif
>   };
> 
>   static bool fred_setup_done __initdata;
> diff --git a/arch/x86/include/asm/idtentry.h 
> b/arch/x86/include/asm/idtentry.h
> index 749c7411d2f1..758f6a2838a8 100644
> --- a/arch/x86/include/asm/idtentry.h
> +++ b/arch/x86/include/asm/idtentry.h
> @@ -745,10 +745,6 @@ DECLARE_IDTENTRY_SYSVEC(IRQ_WORK_VECTOR,        
> sysvec_irq_work);
>   DECLARE_IDTENTRY_SYSVEC(POSTED_INTR_VECTOR,        
> sysvec_kvm_posted_intr_ipi);
>   DECLARE_IDTENTRY_SYSVEC(POSTED_INTR_WAKEUP_VECTOR,    
> sysvec_kvm_posted_intr_wakeup_ipi);
>   DECLARE_IDTENTRY_SYSVEC(POSTED_INTR_NESTED_VECTOR,    
> sysvec_kvm_posted_intr_nested_ipi);
> -#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
> 
>   #if IS_ENABLED(CONFIG_HYPERV)
> 
> and it seems to be a net improvement to me.  The #ifs match in
> the .h and .c files, and there are no unnecessary initializers
> in the sysvec_table.
> 

I somehow get an impression that the x86 maintainers don't like #ifs in
the .c files, but I could be just wrong.

Thanks!
     Xin