Re: [PATCHv3 06/14] x86/kexec: Keep CR4.MCE set during kexec for TDX guest

[Peter Zijlstra]

On Wed, Nov 15, 2023 at 03:00:36PM +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 | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/arch/x86/kernel/relocate_kernel_64.S b/arch/x86/kernel/relocate_kernel_64.S
> index 56cab1bb25f5..bea89814b48e 100644
> --- a/arch/x86/kernel/relocate_kernel_64.S
> +++ b/arch/x86/kernel/relocate_kernel_64.S
> @@ -145,11 +145,16 @@ 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 "jmp 1f", "", X86_FEATURE_TDX_GUEST
> +	orl	$X86_CR4_MCE, %eax
>  1:

	ALTERNATIVE "", "orl $X86_CR4_MCE, %eax", X86_FEATURE_TDX_GUEST

?

>  	movq	%rax, %cr4
>  
> -- 
> 2.41.0
>