target/arm: Handle IC IVAU to improve compatibility with JITs

Unlike architectures with precise self-modifying code semantics
(e.g. x86) ARM processors do not maintain coherency for instruction
execution and memory, requiring an instruction synchronization
barrier on every core that will execute the new code, and on many
models also the explicit use of cache management instructions.

While this is required to make JITs work on actual hardware, QEMU
has gotten away with not handling this since it does not emulate
caches, and unconditionally invalidates code whenever the softmmu
or the user-mode page protection logic detects that code has been
modified.

Unfortunately the latter does not work in the face of dual-mapped
code (a common W^X workaround), where one page is executable and
the other is writable: user-mode has no way to connect one with the
other as that is only known to the kernel and the emulated
application.

This commit works around the issue by telling software that
instruction cache invalidation is required by clearing the
CPR_EL0.DIC flag (regardless of whether the emulated processor
needs it), and then invalidating code in IC IVAU instructions.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1034



Co-authored-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: John Högberg <john.hogberg@ericsson.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 168778890374.24232.3402138851538068785-1@git.sr.ht
[PMM: removed unnecessary AArch64 feature check; moved
 "clear CTR_EL1.DIC" code up a bit so it's not in the middle
 of the vfp/neon related tests]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>