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    Subject[PATCHv9.1 05/17] x86/kexec: Keep CR4.MCE set during kexec for TDX guest
    Date
    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.shutemov@linux.intel.com>
    ---

    v9.1:
    - Preserve CR4.MCE instead of setting it unconditionally.

    ---
    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
    1:
    movq %rax, %cr4

    --
    2.43.0

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