[Qemu-commits] [qemu/qemu] c5ffd1: Revert "cputlb: Restrict SavedIOTLB to system emul...

[Peter Maydell]

  Branch: refs/heads/staging
  Home:   https://github.com/qemu/qemu
  Commit: c5ffd16ba4c8fd3601742cc9d2b3cff03995dd5d
      
https://github.com/qemu/qemu/commit/c5ffd16ba4c8fd3601742cc9d2b3cff03995dd5d
  Author: Peter Maydell <peter.maydell@linaro.org>
  Date:   2023-06-21 (Wed, 21 Jun 2023)

  Changed paths:
    M include/hw/core/cpu.h

  Log Message:
  -----------
  Revert "cputlb: Restrict SavedIOTLB to system emulation"

This reverts commit d7ee93e24359703debf4137f4cc632563aa4e8d1.

That commit tries to make a field in the CPUState struct not be
present when CONFIG_USER_ONLY is set.  Unfortunately, you can't
conditionally omit fields in structs like this based on ifdefs that
are set per-target.  If you try it, then code in files compiled
per-target (where CONFIG_USER_ONLY is or can be set) will disagree
about the struct layout with files that are compiled once-only (where
this kind of ifdef is never set).

This manifests specifically in 'make check-tcg' failing, because code
in cpus-common.c that sets up the CPUState::cpu_index field puts it
at a different offset from the code in plugins/core.c in
qemu_plugin_vcpu_init_hook() which reads the cpu_index field.  The
latter then hits an assert because from its point of view every
thread has a 0 cpu_index. There might be other weird behaviour too.

Mostly we catch this kind of bug because the CONFIG_whatever is
listed in include/exec/poison.h and so the reference to it in
build-once source files will then cause a compiler error.
Unfortunately CONFIG_USER_ONLY is an exception to that: we have some
places where we use it in "safe" ways in headers that will be seen by
once-only source files (e.g.  ifdeffing out function prototypes) and
it would be a lot of refactoring to be able to get to a position
where we could poison it.  This leaves us in a "you have to be
careful to walk around the bear trap" situation...

Fixes: d7ee93e243597 ("cputlb: Restrict SavedIOTLB to system emulation")
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20230620175712.1331625-1-peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>