Michael Ellerman [off-list ref] writes:

Nicholas Piggin [off-list ref] writes:

quoted

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 >_<

cheers