On Thu, Aug 06, 2020 at 03:32:25PM +1000, Michael Ellerman wrote: > That brings with it a bunch of problems, such as existing software that > has been developed/configured for Power8 and expects to see SMT8. > > We also allow LPARs to be live migrated from Power8 to Power9 (and back), so > maintaining the illusion of SMT8 is considered a requirement to make that > work.
So how does that work if the kernel booted on P9 and demuxed the SMT8 into 2xSMT4? If you migrate that state onto a P8 with actual SMT8 you're toast again. > Yeah I agree the naming is confusing. > > Let's call them "SMT4 cores" and "SMT8 cores"? Works for me, thanks! > The problem is we are already lying to userspace, because firmware lies to us. > > ie. the firmware on these systems shows us an SMT8 core, and so current > kernels > show SMT8 to userspace. I don't think we can realistically change that fact > now, > as these systems are already out in the field. > > What this patch tries to do is undo some of the mess, and at least give the > scheduler the right information. What a mess... I think it depends on what you do with that P9 to P8 migration case. Does it make sense to have a "p8_compat" boot arg for the case where you want LPAR migration back onto P8 systems -- in which case it simply takes the firmware's word as gospel and doesn't untangle things, because it can actually land on a P8.