Re: [PATCHv10 05/18] x86/kexec: Keep CR4.MCE set during kexec for TDX guest

[Borislav Petkov]

On Tue, Apr 09, 2024 at 06:26:05PM +0300, Kirill A. Shutemov wrote:
> From 6be428e3b1c6fb494b2c48ba6a7c133514a0b2b4 Mon Sep 17 00:00:00 2001
> From: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
> Date: Fri, 10 Feb 2023 12:53:11 +0300
> Subject: [PATCHv10.1 05/18] x86/kexec: Keep CR4.MCE set during kexec for TDX guest
> 
> Depending on setup, TDX guests might be allowed to clear CR4.MCE.
> Attempt to clear it leads to #VE.
> 
> Use alternatives to keep the flag during kexec for TDX guests.
> 
> The change doesn't affect non-TDX-guest environments.

This is all fine and dandy but nothing explains *why* TDX needs this
special dance.

Why can't TDX do the usual CR4.MCE diddling like the normal kernel
during init and needs to do that here immediately?

> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> ---
>  arch/x86/kernel/relocate_kernel_64.S | 15 +++++++++------
>  1 file changed, 9 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/x86/kernel/relocate_kernel_64.S b/arch/x86/kernel/relocate_kernel_64.S
> index 56cab1bb25f5..90246d544eb1 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>
> @@ -143,14 +145,15 @@ 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 allowed in TDX guests, depending on setup.

			 ... might not be allowed ...

> +	 *  - physical address extension enabled
>  	 */
> -	movl	$X86_CR4_PAE, %eax
> -	testq	$X86_CR4_LA57, %r13
> -	jz	1f
> -	orl	$X86_CR4_LA57, %eax
> -1:
> +	movl	$X86_CR4_LA57, %eax
> +	ALTERNATIVE "", __stringify(orl $X86_CR4_MCE, %eax), X86_FEATURE_TDX_GUEST
> +	andl	%r13d, %eax

%r13 needs a comment here that it contains %cr4 read above in
relocate_kernel()

> +	orl	$X86_CR4_PAE, %eax
>  	movq	%rax, %cr4
>  
>  	jmp 1f
> -- 
>   Kiryl Shutsemau / Kirill A. Shutemov

-- 
Regards/Gruss,
    Boris.

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