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

[Huang, Kai]

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@xxxxxxxxx>