Re: [PATCHv4 06/14] x86/kexec: Keep CR4.MCE set during kexec for TDX guest

[Huang, Kai]

On Tue, 2023-12-05 at 03:45 +0300, Kirill A. Shutemov wrote:
> TDX guests are not allowed to clear CR4.MCE. Attempt to clear it leads
> to #VE.
> 
> Use alternatives to keep the flag during kexec for TDX guests.
> 
> The change doesn't affect non-TDX-guest environments.
> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> Reviewed-by: Kai Huang <kai.huang@xxxxxxxxx>
> ---
>  arch/x86/kernel/relocate_kernel_64.S | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/arch/x86/kernel/relocate_kernel_64.S b/arch/x86/kernel/relocate_kernel_64.S
> index 56cab1bb25f5..cd6a53667c6b 100644
> --- a/arch/x86/kernel/relocate_kernel_64.S
> +++ b/arch/x86/kernel/relocate_kernel_64.S
> @@ -145,12 +145,15 @@ 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. Clearing MCE is not allowed
> +	 *    in TDX guests.
>  	 */
>  	movl	$X86_CR4_PAE, %eax
>  	testq	$X86_CR4_LA57, %r13
>  	jz	1f
>  	orl	$X86_CR4_LA57, %eax
>  1:
> +	ALTERNATIVE "", __stringify(orl $X86_CR4_MCE, %eax), X86_FEATURE_TDX_GUEST
>  	movq	%rax, %cr4
>  
>  	jmp 1f

Nit:

It seems <asm/alternative.h> isn't included in relocate_kernel_64.S.  Maybe it's
better to do it explicitly.

Maybe even better to explicitly include <linux/stringify.h> too, but I see
<asm/alternative.h> already does that.