Re: [PATCH] RISC-V: Add support for Ztso

[Palmer Dabbelt]

On Thu, 29 Sep 2022 12:16:48 PDT (-0700), dgilbert@redhat.com wrote:
> * Palmer Dabbelt (palmer@rivosinc.com) wrote:
> > Ztso, the RISC-V extension that provides the TSO memory model, was
> > recently frozen.  This provides support for Ztso on targets that are
> > themselves TSO.
> >
> > Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>
> >
> > ---
>
> > diff --git a/tcg/i386/tcg-target.h b/tcg/i386/tcg-target.h
> > index 00fcbe297d..2a43d54fcd 100644
> > --- a/tcg/i386/tcg-target.h
> > +++ b/tcg/i386/tcg-target.h
> > @@ -236,6 +236,7 @@ static inline void tb_target_set_jmp_target(uintptr_t 
> > tc_ptr, uintptr_t jmp_rx,
> >  #include "tcg/tcg-mo.h"
> >
> >  #define TCG_TARGET_DEFAULT_MO (TCG_MO_ALL & ~TCG_MO_ST_LD)
> > +#define TCG_TARGET_SUPPORTS_MCTCG_RVTSO 1
>
> Is x86's brand of memory ordering strong enough for Ztso?
> I thought x86 had an optimisation where it was allowed to store forward
> within the current CPU causing stores not to be quite strictly ordered.

I'm actually not sure: my understanding of the Intel memory model was 
that there's a bunch of subtle bits that don't match the various TSO 
formalizations, but the RISC-V folks are pretty adamant that Intel is 
exactly TSO.  I've gotten yelled at enough times on this one that I kind 
of just stopped caring, but that's not a good reason to have broken code 
so I'm happy to go fix it.

That said, when putting together the v2 (which has TCG barriers in the 
RISC-V front-end) I couldn't even really figure out how the TCG memory 
model works in any formal capacity -- I essentially just added the 
fences necessary for Ztso on RVWMO, but that's not a good proxy for Ztso 
on arm64 (and I guess not on x86, either).  Also happy to go take a 
crack at that one, but I'm not really a formal memory model person so it 
might not be the best result.

> Dave
>
> >  #define TCG_TARGET_HAS_MEMORY_BSWAP  have_movbe
> >
> > diff --git a/tcg/s390x/tcg-target.h b/tcg/s390x/tcg-target.h
> > index 23e2063667..f423c124a0 100644
> > --- a/tcg/s390x/tcg-target.h
> > +++ b/tcg/s390x/tcg-target.h
> > @@ -171,6 +171,7 @@ extern uint64_t s390_facilities[3];
> >  #define TCG_TARGET_HAS_MEMORY_BSWAP   1
> >
> >  #define TCG_TARGET_DEFAULT_MO (TCG_MO_ALL & ~TCG_MO_ST_LD)
> > +#define TCG_TARGET_SUPPORTS_MCTCG_RVTSO 1
> >
> >  static inline void tb_target_set_jmp_target(uintptr_t tc_ptr, uintptr_t jmp_rx,
> >                                              uintptr_t jmp_rw, uintptr_t addr)
> > --
> > 2.34.1