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

["Philippe Mathieu-Daudé" <philmd@linaro.org>]

On 21/6/23 11:39, Philippe Mathieu-Daudé wrote:
> On 21/6/23 07:19, Richard Henderson wrote:
>> 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).
> 
> Oops, sorry.
> 
>>> 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.
> 
> Why isn't this covered by CI, and where could we add a such check?

Actually it is covered and failed on staging:
https://gitlab.com/qemu-project/qemu/-/jobs/4503766933

Anyhow, sorry for the mess.

>>> 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.
> 
> Thanks both.
>