Re: [Qemu-devel] [Qemu-ppc] [PATCH] target/ppc/excp_helper: Take BQL before calling cpu_interrupt()

["Alex Bennée" <alex.bennee@linaro.org>]

Nikunj A Dadhania <nikunj@linux.vnet.ibm.com> writes:

> Alex Bennée <alex.bennee@linaro.org> writes:
>
>> Thomas Huth <thuth@redhat.com> writes:
>>
>>> Since the introduction of MTTCG, using the msgsnd instruction
>>> abort()s if being called without holding the BQL. So let's protect
>>> that part of the code now with qemu_mutex_lock_iothread().
>>>
>>> Buglink: https://bugs.launchpad.net/qemu/+bug/1694998
>>> Signed-off-by: Thomas Huth <thuth@redhat.com>
>>
>> Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
>>
>> p.s. I was checking the ppc code for other CPU_FOREACH patterns and I
>> noticed the tlb_flush calls could probably use the tlb_flush_all_cpus
>> API instead of manually looping themselves.
>
> Will that be synchronous call? In PPC, we do lazy tlb flush, the tlb
> flushes are batched until a synchronization point (for optimization).

No by default the non-synced flushes will occur at the end of the
current executing block (cpu->exit_request is set and the work is done
when we exit the run-loop).

> The batching is achieved using a tlb_need_flush (global/local) and when
> there is isync/ptesync or an exception, the actual flush is done. At
> this point we need to make sure that the flush is synchronous.

If you want to ensure the flush is synchronous you need to call the
_all_cpus_synced variants and do a cpu_loop_exit in your helper. This
ensures that all the flushes queued up will be executed before execution
starts at the next PC of the calling thread.

>
>> You should also double check the semantics to make sure none of them
>> need to use the _synced variant and a cpu_exit if the flush needs to
>> complete w.r.t the originating CPU.
>
> Regards,
> Nikunj


--
Alex Bennée