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Message-ID: <5d7dca1e-5889-4440-b3a0-48610f11200e@zytor.com>
Date: Mon, 3 Jun 2024 07:43:06 -0700
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Subject: Re: [PATCHv11 05/19] x86/relocate_kernel: Use named labels for less
 confusion
To: Nikolay Borisov <nik.borisov@suse.com>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
        Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org
Cc: "Rafael J. Wysocki" <rafael@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Adrian Hunter <adrian.hunter@intel.com>,
        Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>,
        Elena Reshetova <elena.reshetova@intel.com>,
        Jun Nakajima <jun.nakajima@intel.com>,
        Rick Edgecombe <rick.p.edgecombe@intel.com>,
        Tom Lendacky <thomas.lendacky@amd.com>,
        "Kalra, Ashish"
 <ashish.kalra@amd.com>,
        Sean Christopherson <seanjc@google.com>,
        "Huang, Kai" <kai.huang@intel.com>, Ard Biesheuvel <ardb@kernel.org>,
        Baoquan He <bhe@redhat.com>, "K. Y. Srinivasan" <kys@microsoft.com>,
        Haiyang Zhang <haiyangz@microsoft.com>, kexec@lists.infradead.org,
        linux-hyperv@vger.kernel.org, linux-acpi@vger.kernel.org,
        linux-coco@lists.linux.dev, linux-kernel@vger.kernel.org
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 <1e1d1aea-7346-4022-9f5f-402d171adfda@suse.com>
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From: "H. Peter Anvin" <hpa@zytor.com>
In-Reply-To: <1e1d1aea-7346-4022-9f5f-402d171adfda@suse.com>
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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