[PATCHv9 05/17] x86/kexec: Keep CR4.MCE set during kexec for TDX guest

[Xiaoyao Li]

On 3/25/2024 6:38 PM, Kirill A. Shutemov wrote:
> TDX guests are not allowed to clear CR4.MCE. Attempt to clear it leads
> to #VE.

Will we consider making it more safe and compatible for future to guard 
against X86_FEATURE_MCE as well?

If in the future, MCE becomes configurable for TD guest, then CR4.MCE 
might not be fixed1.

> 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 at linux.intel.com>
> Reviewed-by: Kai Huang <kai.huang at intel.com>
> Reviewed-by: Thomas Gleixner <tglx at linutronix.de>
> ---
>   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..e144bcf60cbe 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>
> @@ -145,12 +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. 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