On Tue, Apr 09, 2024, Kirill A. Shutemov wrote:
> 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.
>
> Signed-off-by: Kirill A. Shutemov <kirill.shute...@linux.intel.com>
> ---
> arch/x86/kernel/relocate_kernel_64.S | 8 ++++++++
> 1 file changed, 8 insertions(+)
>
> 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
The TEST+Jcc+OR sequences are rather odd, and require way more instructions and
thus way more copy+paste than is necessary.
movl $X86_CR4_LA57, %eax
ALTERNATIVE "", __stringify(orl $X86_CR4_MCE, %eax),
X86_FEATURE_TDX_GUEST
andl %r13d, %eax
orl $X86_CR4_PAE, %eax
movq %rax, %cr4
Then preserving new bits unconditionally only requires adding the flag to the
initial move, and feature-dependent bits only need a single ALTERNATIVE line.
And there's no branches, blazing fast kexec! ;-)
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