[PATCHv10.1 05/18] x86/kexec: Keep CR4.MCE set during kexec for TDX guest

[Kirill A. Shutemov]

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@xxxxxxxxxxxxxxx>
---
 arch/x86/kernel/relocate_kernel_64.S | 15 +++++++++------
 1 file changed, 9 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kernel/relocate_kernel_64.S b/arch/x86/kernel/relocate_kernel_64.S
index 56cab1bb25f5..90246d544eb1 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>
@@ -143,14 +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, if it was enabled before.
+	 *    Clearing MCE might not allowed in TDX guests, depending on setup.
+	 *  - physical address extension enabled
 	 */
-	movl	$X86_CR4_PAE, %eax
-	testq	$X86_CR4_LA57, %r13
-	jz	1f
-	orl	$X86_CR4_LA57, %eax
-1:
+	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
 
 	jmp 1f
-- 
  Kiryl Shutsemau / Kirill A. Shutemov