tools/objtool: Check for use of the ENQCMD instruction in the kernel

The ENQCMD instruction implicitly accesses the PASID_MSR to fill in the
pasid field of the descriptor being submitted to an accelerator. But
there is no precise (and stable across kernel changes) point at which
the PASID_MSR is updated from the value for one task to the next.

Kernel code that uses accelerators must always use the ENQCMDS instruction
which does not access the PASID_MSR.

Check for use of the ENQCMD instruction in the kernel and warn on its
usage.

Signed-off-by: Fenghua Yu <[email protected]>
Signed-off-by: Borislav Petkov <[email protected]>
Reviewed-by: Tony Luck <[email protected]>
Acked-by: Josh Poimboeuf <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Peter Zijlstra <[email protected]>

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