linux-kernel - Re: [PATCH 1/2] x86/fred: Always install system interrupt handler into IDT

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Date: Tue, 25 Jun 2024 19:01:17 +0200
From: Thomas Gleixner <tglx@...utronix.de>
To: Xin Li <xin@...or.com>, Hou Wenlong <houwenlong.hwl@...group.com>,
 linux-kernel@...r.kernel.org
Cc: Lai Jiangshan <jiangshan.ljs@...group.com>, Ingo Molnar
 <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, Dave Hansen
 <dave.hansen@...ux.intel.com>, x86@...nel.org, "H. Peter Anvin"
 <hpa@...or.com>, Xin Li <xin3.li@...el.com>, Jacob Pan
 <jacob.jun.pan@...ux.intel.com>, Rick Edgecombe
 <rick.p.edgecombe@...el.com>, Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH 1/2] x86/fred: Always install system interrupt handler
 into IDT

On Tue, Jun 25 2024 at 09:30, Xin Li wrote:

> On 6/25/2024 5:31 AM, Thomas Gleixner wrote:
>> On Tue, Jun 25 2024 at 02:19, Xin Li wrote:
>>> On 6/21/2024 6:12 AM, Hou Wenlong wrote:
>>>> diff --git a/arch/x86/include/asm/idtentry.h b/arch/x86/include/asm/idtentry.h
>>>> index d4f24499b256..daee9f7765bc 100644
>>>> --- a/arch/x86/include/asm/idtentry.h
>>>> +++ b/arch/x86/include/asm/idtentry.h
>>>> @@ -470,8 +470,7 @@ static inline void fred_install_sysvec(unsigned int vector, const idtentry_t fun
>>>>    #define sysvec_install(vector, function) {				\
>>>>    	if (cpu_feature_enabled(X86_FEATURE_FRED))			\
>>>>    		fred_install_sysvec(vector, function);			\
>>>> -	else								\
>>>> -		idt_install_sysvec(vector, asm_##function);		\
>>>
>>> empty line, it improves readability.
>>>
>>> And please put a comment here to explain why this is unconditionally
>>> done for IDT.
>> 
>> Wait. If we need this during early boot, then why don't we enable FRED
>> earlier?
>
> Unconditionally call idt_install_sysvec() is still needed, right?

Not when we enable FRED early enough.

> But this sounds a smart move to me!  Because it helps to deal with not
> only the initialization order issue, but also the following cases in a
> longer term:
>
> 1) BIOS enables FRED and keeps it enabled when transferring control to
>     Linux.  Then we just need to disable FRED when it is disabled in the
>     kernel command line.
>
> 2) IDT support is removed from a kernel config thus a kernel binary, say
>     a new kernel config CONFIG_X86_IDT is added and set to N.
>
> And we need to:
>
> 1) Find a place to enable FRED as early as possible if not yet enabled.
>
> 2) Disable FRED when fred=off is in the kernel command line.
>
> Anything I missed?

No.

Thanks,

        tglx