Re: [Qemu-devel] [PATCH v6 19/19] tcg: enable MTTCG by default for ARM on x86 hosts

[Richard Henderson]

On 11/10/2016 07:13 PM, Alex Bennée wrote:
> Richard Henderson <address@hidden> writes:
>
> > On 11/09/2016 03:57 PM, Alex Bennée wrote:
> > > This enables the multi-threaded system emulation by default for ARMv7
> > > and ARMv8 guests using the x86_64 TCG backend. This means:
> > >
> > >   - The x86_64 TCG backend supports cmpxchg based atomic ops
> > >   - The x86_64 TCG backend emits barriers for barrier ops
> >
> > What tcg backend doesn't support what we need?  For a weakly ordered target,
> > any of our hosts should work.
>
> True, but this comes with certification that I've tested it. But you are
> right adding this to configure is fugly. Should I just drop the backend
> config symbol requirement totally?

I was thinking that a good backend config symbol would somehow indicate the 
memory ordering strength of the host.  Preferably in such a way that we can 
tell that host >= guest.

I dunno if we assign ordinal numbers in some arbitrary way, or try something 
more complex such as

  x86_64)
    HOST_MTTCG_MO='TCG_MO_ALL & ~TCG_MO_LD_ST'

Or maybe put this in tcg/*/tcg-target.h in preference to configure.

Then enable mttcg if the host memory-order is a superset of the guest,

    (GUEST_MTTCG_MO & ~HOST_MTTCG_MO) == 0



r~