linux-kernel - [tip: x86/cc] x86/kexec: Keep CR4.MCE set during kexec for TDX guest

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Message-ID: <171871930225.10875.13385118574589510822.tip-bot2@tip-bot2>
Date: Tue, 18 Jun 2024 14:01:42 -0000
From: "tip-bot2 for Kirill A. Shutemov" <tip-bot2@...utronix.de>
To: linux-tip-commits@...r.kernel.org
Cc: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
 "Borislav Petkov (AMD)" <bp@...en8.de>, x86@...nel.org,
 linux-kernel@...r.kernel.org
Subject: [tip: x86/cc] x86/kexec: Keep CR4.MCE set during kexec for TDX guest

The following commit has been merged into the x86/cc branch of tip:

Commit-ID:     de60613173dfd75a10f6aa8e001bbcafa242e623
Gitweb:        https://git.kernel.org/tip/de60613173dfd75a10f6aa8e001bbcafa242e623
Author:        Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
AuthorDate:    Fri, 14 Jun 2024 12:58:51 +03:00
Committer:     Borislav Petkov (AMD) <bp@...en8.de>
CommitterDate: Mon, 17 Jun 2024 17:45:50 +02:00

x86/kexec: Keep CR4.MCE set during kexec for TDX guest

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.

Therefore, make sure the CR4.MCE setting is preserved over kexec too and avoid
raising any #VEs.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
Signed-off-by: Borislav Petkov (AMD) <bp@...en8.de>
Link: https://lore.kernel.org/r/20240614095904.1345461-7-kirill.shutemov@linux.intel.com
---
 arch/x86/kernel/relocate_kernel_64.S | 17 ++++++++++-------
 1 file changed, 10 insertions(+), 7 deletions(-)

diff --git a/arch/x86/kernel/relocate_kernel_64.S b/arch/x86/kernel/relocate_kernel_64.S
index 8b8922d..042c9a0 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>
@@ -145,14 +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, if it was enabled before.
+	 *    Clearing MCE might not be allowed in TDX guests, depending on setup.
+	 *
+	 * Use R13 that contains the original CR4 value, read in relocate_kernel().
+	 * PAE is always set in the original CR4.
 	 */
-	movl	$X86_CR4_PAE, %eax
-	testq	$X86_CR4_LA57, %r13
-	jz	.Lno_la57
-	orl	$X86_CR4_LA57, %eax
-.Lno_la57:
-
-	movq	%rax, %cr4
+	andl	$(X86_CR4_PAE | X86_CR4_LA57), %r13d
+	ALTERNATIVE "", __stringify(orl $X86_CR4_MCE, %r13d), X86_FEATURE_TDX_GUEST
+	movq	%r13, %cr4
 
 	/* Flush the TLB (needed?) */
 	movq	%r9, %cr3