Re: [PATCH] Revert "cputlb: Restrict SavedIOTLB to system emulation"

[Richard Henderson <richard.henderson@linaro.org>]

On 6/20/23 19:57, Peter Maydell wrote:
> 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>
> ---
>   include/hw/core/cpu.h | 6 ++----
>   1 file changed, 2 insertions(+), 4 deletions(-)

Ho hum, thanks.  I'll apply this directly.

r~