[PATCHv9 05/17] x86/kexec: Keep CR4.MCE set during kexec for TDX guest
Huang, Kai
kai.huang at intel.com
Wed Apr 3 14:40:34 PDT 2024
On 3/04/2024 4:42 am, Kirill A. Shutemov wrote:
> 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.
Nit: Perhaps we can just call out:
Clearing MCE is not allowed if it _was_ enabled before.
Which is always true I suppose.
> */
> 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
>
Anyway,
Reviewed-by: Kai Huang <kai.huang at intel.com>
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