Re: [PATCHv10 05/18] x86/kexec: Keep CR4.MCE set during kexec for TDX guest

[Sean Christopherson <seanjc@google.com>]

On Tue, Apr 09, 2024, Kirill A. Shutemov wrote:
> Depending on setup, TDX guests might be 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@linux.intel.com>
> ---
>  arch/x86/kernel/relocate_kernel_64.S | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/arch/x86/kernel/relocate_kernel_64.S b/arch/x86/kernel/relocate_kernel_64.S
> index 56cab1bb25f5..8e2037d78a1f 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>
> @@ -145,11 +147,17 @@ 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 allowed in TDX guests, depending on setup.
>  	 */
>  	movl	$X86_CR4_PAE, %eax
>  	testq	$X86_CR4_LA57, %r13
>  	jz	1f
>  	orl	$X86_CR4_LA57, %eax
> +1:
> +	testq	$X86_CR4_MCE, %r13
> +	jz	1f
> +	ALTERNATIVE "", __stringify(orl $X86_CR4_MCE, %eax), X86_FEATURE_TDX_GUEST

The TEST+Jcc+OR sequences are rather odd, and require way more instructions and
thus way more copy+paste than is necessary.

	movl	$X86_CR4_LA57, %eax
	ALTERNATIVE "", __stringify(orl $X86_CR4_MCE, %eax), X86_FEATURE_TDX_GUEST
	andl	%r13d, %eax
	orl	$X86_CR4_PAE, %eax
	movq	%rax, %cr4

Then preserving new bits unconditionally only requires adding the flag to the
initial move, and feature-dependent bits only need a single ALTERNATIVE line.

And there's no branches, blazing fast kexec! ;-)