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 07:31:46 +0100
From: Paolo Bonzini <pbonzini@...hat.com>
To: 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 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), 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.

Paolo