Re: [Qemu-devel] [PATCH 6/8] tcg/tci: disable MTTCG if TCI is enabled

[Philippe Mathieu-Daudé]

On 07/26/2017 05:02 AM, Alex Bennée wrote:
> Philippe Mathieu-Daudé <address@hidden> writes:
>
> > Hi Peter,
> >
> > I think this patch belongs to 2.10, as there is no time to fix TCI +
> > MTTCG. Should I RESEND it alone with "for 2.10" subject?
> >
> > One other option might be disable TCI if MTTCG enabled, but there is
> > no ./configure option for MTTCG while there is for TCI.
> >
> > Regards,
> >
> > Phil.
> >
> > On 06/28/2017 10:02 PM, Philippe Mathieu-Daudé wrote:
> > > TCI + MTTCG cause strange errors...
> > >
> > >     $ arm-softmmu/qemu-system-arm -machine raspi2 -cpu cortex-a7 -smp 4 -accel 
> > > tcg,thread=multi -kernel kernel7.img
> > >     qemu-system-arm: Guest expects a stronger memory ordering than the host 
> > > provides
> > >     This may cause strange/hard to debug errors
> > >     Segmentation fault (core dumped)
>
> So this isn't TCI enabling MTTCG by accident - this is the user forcing
> it when it wouldn't otherwise be enabled. Hence the scary warning...

Hmmm this is the user enabling TCI, I'm not sure about "forcing it".
Now MTTCG is enforced by the ./configure and there is no option to 
disable it.

I posted this patch after realizing there are more TCI users/testers 
than just Stefan Weil, Jaroslaw Pelczar and me. Peter commented about 
http://qira.me/ which doesn't look very active.
(see this thread: 
http://lists.nongnu.org/archive/html/qemu-devel/2017-06/threads.html#06528 )

Thinking about eventual downstream project using the TCI part of QEMU 
and trying to stay up-to-date with upstream, this patch might save 
headaches.

Since there is no formal yes/no to this patch I'm still wondering if I 
should track it or no.

Regards,

Phil.

> > > Signed-off-by: Philippe Mathieu-Daudé <address@hidden>
> > > ---
> > >    configure | 6 +++++-
> > >    1 file changed, 5 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/configure b/configure
> > > index c571ad14e5..510f443e06 100755
> > > --- a/configure
> > > +++ b/configure
> > > @@ -6225,7 +6225,11 @@ fi
> > >    if test "$target_softmmu" = "yes" ; then
> > >      echo "CONFIG_SOFTMMU=y" >> $config_target_mak
> > >      if test "$mttcg" = "yes" ; then
> > > -    echo "TARGET_SUPPORTS_MTTCG=y" >> $config_target_mak
> > > +    if test "$tcg_interpreter" = "yes" ; then
> > > +        echo "TCI enabled, disabling MTTCG"
> > > +    else
> > > +        echo "TARGET_SUPPORTS_MTTCG=y" >> $config_target_mak
> > > +    fi
> > >      fi
> > >    fi
> > >    if test "$target_user_only" = "yes" ; then
>
>
> --
> Alex Bennée