diff mbox series

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

Message ID 20240301185618.19663-2-bp@alien8.de
State New
Headers
Series x86/kexec: Revert 5level dynamic switching |

Commit Message

Borislav Petkov March 1, 2024, 6:56 p.m. UTC 
  From: "Borislav Petkov (AMD)" <bp@alien8.de>

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.

But I'm open to corrections.

Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
---
 arch/x86/kernel/kexec-bzimage64.c | 5 -----
 1 file changed, 5 deletions(-)

Comments

Baoquan He March 4, 2024, 10:51 a.m. UTC | #1 
On 03/01/24 at 07:56pm, Borislav Petkov wrote:
> From: "Borislav Petkov (AMD)" <bp@alien8.de>
> 
> 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@alien8.de>
> ---
>  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
>
Borislav Petkov March 4, 2024, 11:11 a.m. UTC | #2 
On Mon, Mar 04, 2024 at 06:51:26PM +0800, Baoquan He wrote:
> It's not true. Customer may want to try to load a different kernel if

"may want" is one of those hypothetical things which we don't do. If we
have to support everything a customer *may* want, then the kernel will
be a madness.

Also, you do realize that the kernel doesn't care about "customers",
right?

And the question is, how *sensible* is such a use case?

In my experience, not at all. You simply take the same kernel or a very
similar one and kexec it.

> they have taken many testings and trust that kdump kernel, or for
> debugging.

Yes, and those kernels will have 5level too. Practically, distros must
enable 5level support in their kernels in order to support modern hw.

> 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 can't parse that example.

Btw, kexec tools don't use those XLF_5LEVEL* flags bits either. Which
basically means we don't really need them.

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

See above.

And that customer can still use the old distro kernels which have those
flags.

The point here is, going forward, 5level becomes ubiquitous and will be
even more tightly integrated in the kernel so that it'll become just
another default feature which is either there or not.

So the distinction is going away and the flags can go too.

Thx.
diff mbox series

Patch

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;