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

[Nikolay Borisov]

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