Re: [RFC PATCH 1/2] Revert "x86/kexec/64: Prevent kexec from 5-level paging to a 4-level only kernel"

[Baoquan He]

On 03/01/24 at 07:56pm, Borislav Petkov wrote:
> From: "Borislav Petkov (AMD)" <bp@xxxxxxxxx>
> 
> This reverts commit ee338b9ee2822e65a85750da6129946c14962410.
> 
> This whole dynamic switching support is silly. I don't see a use case
> where one would use an old kernel with CONFIG_X86_5LEVEL disabled to
> kexec into. I.e., you use pretty much the same kernel.

It's not true. Customer may want to try to load a different kernel if
they have taken many testings and trust that kdump kernel, or for
debugging. The similar for kexec reboot into 2nd kernel. We don't
enforce kexec/kdump to work on the same kernel as the 1st kernel. With
the fail and message, user can take measure to avoid that. it's better
the failure is encountered when failing to jump to kexec/kdump kernel.

I remmeber we have use case where customer used kdump kernel different
than the 1st kernel. While I don't remember why.

> 
> But I'm open to corrections.
> 
> Signed-off-by: Borislav Petkov (AMD) <bp@xxxxxxxxx>
> ---
>  arch/x86/kernel/kexec-bzimage64.c | 5 -----
>  1 file changed, 5 deletions(-)
> 
> diff --git a/arch/x86/kernel/kexec-bzimage64.c b/arch/x86/kernel/kexec-bzimage64.c
> index cde167b0ea92..4f2e47338b7f 100644
> --- a/arch/x86/kernel/kexec-bzimage64.c
> +++ b/arch/x86/kernel/kexec-bzimage64.c
> @@ -375,11 +375,6 @@ static int bzImage64_probe(const char *buf, unsigned long len)
>  		return ret;
>  	}
>  
> -	if (!(header->xloadflags & XLF_5LEVEL) && pgtable_l5_enabled()) {
> -		pr_err("bzImage cannot handle 5-level paging mode.\n");
> -		return ret;
> -	}
> -
>  	/* I've got a bzImage */
>  	pr_debug("It's a relocatable bzImage64\n");
>  	ret = 0;
> -- 
> 2.43.0
>