Re: [PATCHv9 05/17] x86/kexec: Keep CR4.MCE set during kexec for TDX guest

[Kirill A. Shutemov]

On Fri, Mar 29, 2024 at 06:48:21PM +0200, Kirill A. Shutemov wrote:
> On Fri, Mar 29, 2024 at 11:21:32PM +0800, Xiaoyao Li wrote:
> > On 3/25/2024 6:38 PM, Kirill A. Shutemov wrote:
> > > TDX guests are not allowed to clear CR4.MCE. Attempt to clear it leads
> > > to #VE.
> > 
> > Will we consider making it more safe and compatible for future to guard
> > against X86_FEATURE_MCE as well?
> > 
> > If in the future, MCE becomes configurable for TD guest, then CR4.MCE might
> > not be fixed1.
> 
> Good point.
> 
> I guess we can leave it clear if it was clear. This should be easy
> enough. But we might want to clear even if was set if clearing is allowed.
> 
> It would require some kind of indication that clearing MCE is fine. We
> don't have such indication yet. Not sure we can reasonably future-proof
> the code at this point.
> 
> But let me think more.

I think I will go with the variant below.

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
 1:
 	movq	%rax, %cr4
 
-- 
  Kiryl Shutsemau / Kirill A. Shutemov