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.
https://people.kernel.org/tglx/notes-about-netiquette
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