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
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