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

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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




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