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

["H. Peter Anvin" <hpa@zytor.com>]

On 6/4/24 08:21, Kirill A. Shutemov wrote:
> 
>  From b45fe48092abad2612c2bafbb199e4de80c99545 Mon Sep 17 00:00:00 2001
> From: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
> Date: Fri, 10 Feb 2023 12:53:11 +0300
> Subject: [PATCHv11.1 06/19] x86/kexec: Keep CR4.MCE set during kexec for TDX guest
> 
> TDX guests run with MCA enabled (CR4.MCE=1b) from the very start. If
> that bit is cleared during CR4 register reprogramming during boot or
> kexec flows, a #VE exception will be raised which the guest kernel
> cannot handle it.
> 
> Therefore, make sure the CR4.MCE setting is preserved over kexec too and
> avoid raising any #VEs.
> 
> The change doesn't affect non-TDX-guest environments.
> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> ---
>   arch/x86/kernel/relocate_kernel_64.S | 17 ++++++++++-------
>   1 file changed, 10 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/x86/kernel/relocate_kernel_64.S b/arch/x86/kernel/relocate_kernel_64.S
> index 085eef5c3904..9c2cf70c5f54 100644
> --- a/arch/x86/kernel/relocate_kernel_64.S
> +++ b/arch/x86/kernel/relocate_kernel_64.S
> @@ -5,6 +5,8 @@
>    */
>   
>   #include <linux/linkage.h>
> +#include <linux/stringify.h>
> +#include <asm/alternative.h>
>   #include <asm/page_types.h>
>   #include <asm/kexec.h>
>   #include <asm/processor-flags.h>
> @@ -145,14 +147,15 @@ SYM_CODE_START_LOCAL_NOALIGN(identity_mapped)
>   	 * Set cr4 to a known state:
>   	 *  - physical address extension enabled
>   	 *  - 5-level paging, if it was enabled before
> +	 *  - Machine check exception on TDX guest, if it was enabled before.
> +	 *    Clearing MCE might not be allowed in TDX guests, depending on setup.
> +	 *
> +	 * Use R13 that contains the original CR4 value, read in relocate_kernel().
> +	 * PAE is always set in the original CR4.
>   	 */
> -	movl	$X86_CR4_PAE, %eax
> -	testq	$X86_CR4_LA57, %r13
> -	jz	.Lno_la57
> -	orl	$X86_CR4_LA57, %eax
> -.Lno_la57:
> -
> -	movq	%rax, %cr4
> +	andl	$(X86_CR4_PAE | X86_CR4_LA57), %r13d
> +	ALTERNATIVE "", __stringify(orl $X86_CR4_MCE, %r13d), X86_FEATURE_TDX_GUEST
> +	movq	%r13, %cr4
>   

If this is the case, I don't really see a reason to clear MCE per se as 
I'm guessing a machine check here will be fatal anyway? It just changes 
the method of death.

Also, is there a reason to save %cr4, run code, and *then* clear the 
relevant bits? Wouldn't it be better to sanitize %cr4 as soon as possible?

	-hpa