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

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Message-ID: <6eb5c1b2-64d2-42fd-81c9-97c1ae65713f@suse.com>
Date: Wed, 12 Jun 2024 15:10:21 +0300
From: Nikolay Borisov <nik.borisov@...e.com>
To: "H. Peter Anvin" <hpa@...or.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 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