linux-kernel - Re: [PATCHv11 05/19] x86/relocate_kernel: Use named labels for less confusion

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Date: Mon, 3 Jun 2024 07:43:06 -0700
From: "H. Peter Anvin" <hpa@...or.com>
To: Nikolay Borisov <nik.borisov@...e.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org
Cc: "Rafael J. Wysocki" <rafael@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Elena Reshetova <elena.reshetova@...el.com>,
        Jun Nakajima <jun.nakajima@...el.com>,
        Rick Edgecombe <rick.p.edgecombe@...el.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        "Kalra, Ashish"
 <ashish.kalra@....com>,
        Sean Christopherson <seanjc@...gle.com>,
        "Huang, Kai" <kai.huang@...el.com>, Ard Biesheuvel <ardb@...nel.org>,
        Baoquan He <bhe@...hat.com>, "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>, kexec@...ts.infradead.org,
        linux-hyperv@...r.kernel.org, linux-acpi@...r.kernel.org,
        linux-coco@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCHv11 05/19] x86/relocate_kernel: Use named labels for less
 confusion

On 5/29/24 03:47, Nikolay Borisov wrote:
>>
>> diff --git a/arch/x86/kernel/relocate_kernel_64.S 
>> b/arch/x86/kernel/relocate_kernel_64.S
>> index 56cab1bb25f5..085eef5c3904 100644
>> --- a/arch/x86/kernel/relocate_kernel_64.S
>> +++ b/arch/x86/kernel/relocate_kernel_64.S
>> @@ -148,9 +148,10 @@ SYM_CODE_START_LOCAL_NOALIGN(identity_mapped)
>>        */
>>       movl    $X86_CR4_PAE, %eax
>>       testq    $X86_CR4_LA57, %r13
>> -    jz    1f
>> +    jz    .Lno_la57
>>       orl    $X86_CR4_LA57, %eax
>> -1:
>> +.Lno_la57:
>> +
>>       movq    %rax, %cr4
>>       jmp 1f
> 
> That jmp 1f becomes redundant now as it simply jumps 1 line below.
> 

Uh... am I the only person to notice that ALL that is needed here is:

	andl $(X86_CR4_PAE|X86_CR4_LA57), %r13d
	movq %r13, %rax

... since %r13 is dead afterwards, and PAE *will* have been set in %r13 
already?

I don't believe that this specific jmp is actually needed -- there are 
several more synchronizing jumps later -- but it doesn't hurt.

However, if the effort is for improving the readability, it might be 
worthwhile to encapsulate the "jmp 1f; 1:" as a macro, e.g. "SYNC_CODE".

	-hpa