[PATCH 4/4] powerpc/64s: Use POWER10 stsync barrier for wmb()

[Joel Stanley]

On Thu, 24 Aug 2023 at 12:12, Michael Ellerman <mpe at ellerman.id.au> wrote:
>
> Michael Ellerman <mpe at ellerman.id.au> writes:
> > Michael Ellerman <mpe at ellerman.id.au> writes:
> >> "Nicholas Piggin" <npiggin at gmail.com> writes:
> >>> On Wed Jun 14, 2023 at 3:56 PM AEST, Michael Ellerman wrote:
> >>>> Michael Ellerman <mpe at ellerman.id.au> writes:
> >>>> > Nicholas Piggin <npiggin at gmail.com> writes:
> >>>> >> The most expensive ordering for hwsync to provide is the store-load
> >>>> >> barrier, because all prior stores have to be drained to the caches
> >>>> >> before subsequent instructions can complete.
> >>>> >>
> >>>> >> stsync just orders stores which means it can just be a barrer that
> >>>> >> goes down the store queue and orders draining, and does not prevent
> >>>> >> completion of subsequent instructions. So it should be faster than
> >>>> >> hwsync.
> >>>> >>
> >>>> >> Use stsync for wmb(). Older processors that don't recognise the SC
> >>>> >> field should treat this as hwsync.
> >>>> >
> >>>> > qemu (7.1) emulating ppc64e does not :/
> >>>> >
> >>>> >   mpic: Setting up MPIC " OpenPIC  " version 1.2 at fe0040000, max 1 CPUs
> >>>> >   mpic: ISU size: 256, shift: 8, mask: ff
> >>>> >   mpic: Initializing for 256 sources
> >>>> >   Oops: Exception in kernel mode, sig: 4 [#1]
> >>>> ..
> >>>> >
> >>>> > I guess just put it behind an #ifdef 64S.
> >>>>
> >>>> That doesn't work because qemu emulating a G5 also doesn't accept it.
> >>>>
> >>>> So either we need to get qemu updated and wait a while for that to
> >>>> percolate, or do some runtime patching of wmbs in the kernel >_<
> >>>
> >>> Gah, sorry. QEMU really should be ignoring reserved fields in
> >>> instructions :(
> >>
> >> Yeah, it's an annoying discrepancy vs real hardware and the ISA.
> >>
> >>> I guess leave it out for now. Should fix QEMU but we probably also need
> >>> to do patching so as not to break older QEMUs.
> >>
> >> I'll plan to take the first 3 patches, they seem OK as-is.
> >
> > I didn't do that in the end, because patch 2 suffers from the same
>                                              ^
>                                              3
> > problem of not working on QEMU.

Did we get a patch to fix this in to Qemu?

Qemu has recently developed a stable tree process, so if we had a
backportable fix we could get it in there too.

Cheers,

Joel