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powerpc/64: use interrupt restart table to speed up return from interrupt
Use the restart table facility to return from interrupt or system calls without disabling MSR[EE] or MSR[RI]. Interrupt return asm is put into the low soft-masked region, to prevent interrupts being processed here, although they are still taken as masked interrupts which causes SRRs to be clobbered, and a pending soft-masked interrupt to require replaying. The return code uses restart table regions to redirct to a fixup handler rather than continue with the exit, if such an interrupt happens. In this case the interrupt return is redirected to a fixup handler which reloads r1 for the interrupt stack and reloads registers and sets state up to replay the soft-masked interrupt and try the exit again. Some types of security exit fallback flushes and barriers are currently unable to cope with reentrant interrupts, e.g., because they store some state in the scratch SPR which would be clobbered even by masked interrupts. For now the interrupts-enabled exits are disabled when these flushes are used. Signed-off-by:Nicholas Piggin <npiggin@gmail.com> [mpe: Guard unused exit_must_hard_disable() as reported by lkp] Signed-off-by: