On 6/4/24 08:21, Kirill A. Shutemov wrote:
From b45fe48092abad2612c2bafbb199e4de80c99545 Mon Sep 17 00:00:00 2001
From: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
Date: Fri, 10 Feb 2023 12:53:11 +0300
Subject: [PATCHv11.1 06/19] 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 it.
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@xxxxxxxxxxxxxxx>
---
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 085eef5c3904..9c2cf70c5f54 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
If this is the case, I don't really see a reason to clear MCE per se as
I'm guessing a machine check here will be fatal anyway? It just changes
the method of death.
Also, is there a reason to save %cr4, run code, and *then* clear the
relevant bits? Wouldn't it be better to sanitize %cr4 as soon as possible?
-hpa
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