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Date: Wed, 12 Jun 2024 15:10:21 +0300
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Subject: Re: [PATCHv11 05/19] x86/relocate_kernel: Use named labels for less
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To: "H. Peter Anvin" <hpa@zytor.com>,
 "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
 Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
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Cc: "Rafael J. Wysocki" <rafael@kernel.org>,
 Peter Zijlstra <peterz@infradead.org>,
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 Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>,
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 Jun Nakajima <jun.nakajima@intel.com>,
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 <ashish.kalra@amd.com>, Sean Christopherson <seanjc@google.com>,
 "Huang, Kai" <kai.huang@intel.com>, Ard Biesheuvel <ardb@kernel.org>,
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From: Nikolay Borisov <nik.borisov@suse.com>
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On 3.06.24 г. 17:43 ч., H. Peter Anvin wrote:
> 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".


The preceding move to CR4 is itself a serializing instruction, no?

> 
>      -hpa