Re: [PATCHv11 06/19] x86/kexec: Keep CR4.MCE set during kexec for TDX guest

["Huang, Kai" <kai.huang@intel.com>]

On Tue, 2024-05-28 at 12:55 +0300, Kirill A. Shutemov wrote:
> 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.

Nit: the ending "it" isn't needed.

> 
> 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>

Reviewed-by: Kai Huang <kai.huang@intel.com>

> ---
>  arch/x86/kernel/relocate_kernel_64.S | 16 ++++++++++------
>  1 file changed, 10 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/x86/kernel/relocate_kernel_64.S b/arch/x86/kernel/relocate_kernel_64.S
> index 085eef5c3904..b668a6be4f6f 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>
> @@ -143,15 +145,17 @@ 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.
> +	 *  - physical address extension enabled
>  	 */
> -	movl	$X86_CR4_PAE, %eax
> -	testq	$X86_CR4_LA57, %r13
> -	jz	.Lno_la57
> -	orl	$X86_CR4_LA57, %eax
> -.Lno_la57:
> +	movl	$X86_CR4_LA57, %eax
> +	ALTERNATIVE "", __stringify(orl $X86_CR4_MCE, %eax), X86_FEATURE_TDX_GUEST
>  
> +	/* R13 contains the original CR4 value, read in relocate_kernel() */
> +	andl	%r13d, %eax
> +	orl	$X86_CR4_PAE, %eax
>  	movq	%rax, %cr4
>  
>  	jmp 1f