Comment 3 for bug 1723984

You shouldn't need to read the MIDR at all.

There are two sensible strategies for software I think:

 (1) trust the architectural statement that v8 implies that the AIFSR and ADFSR both exist -- AIUI both QEMU and the hardware implementations that report 0001 in this MMFR0 field do actually implement those registers, so this is safe.

 (2) read and pay attention to the AuxReg field, by handling 0001 as "only Auxiliary Control Register is supported, AIFSR and ADFSR are not supported". This will work fine too -- on implementations that report 0001 you may be not using the AIFSR/ADFSR but that's ok because on those implementations they only RAZ/WI anyhow so you couldn't do anything interesting with them anyway.

If your code is genuinely v8 only then (1) is easiest. If you also need to support ARMv7 then (2) is best, because 0001 is a permitted value in ID_MMFR0 for an ARMv7 implementation, so you need to handle it regardless of the A53/A57 behaviour.

Neither approach requires detecting and special casing A53/A57 revisions via the MIDR.