Re: mttcg: warning to user about memory ordering

[Pierrick Bouvier]

On 2/22/23 20:14, Richard Henderson wrote:
> On 2/22/23 06:28, Pierrick Bouvier wrote:
> > Hello,
> >
> > When using multi thread tcg, a warning is emitted to user if models for guest 
> > and host are
> > not compatible (like qemu-system-x86_64 running on aarch64 host)
> >
> > accel/tcg/tcg-all.c:
> > 161             if (!check_tcg_memory_orders_compatible()) {
> > 162                 warn_report("Guest expects a stronger memory ordering "
> > 163                             "than the host provides");
> > 164                 error_printf("This may cause strange/hard to debug 
> > errors\n");
> > 165             }
> >
> > However, in tcg/tcg-op.c, all load/stores are emitting memory barriers using
> > tcg_gen_req_mo, which lets think it's already implemented for any host/guest 
> > combination
> > (was added in b32dc3).
> >
> > In your opinion, are there still corner cases that are not covered correctly? 
> > If no,
> > should we drop this scary warning?
>
> See
>
> https://lore.kernel.org/qemu-devel/20210316220735.2048137-1-richard.henderson@linaro.org/
> https://gitlab.com/rth7680/qemu/-/commits/tcg-mb-small/
>
> It never got merged because it didn't actually solve the s390x on aarch64 
> problem.  I
> assume there's something I've missed.

Thanks for the pointers.

> r~