Nicholas Piggin <npig...@gmail.com> writes: > On Mon, 22 May 2017 08:09:19 +1000 > Benjamin Herrenschmidt <b...@au1.ibm.com> wrote: > >> On Mon, 2017-05-22 at 00:00 +1000, Nicholas Piggin wrote: >> > I'd like to take over the r14 register for use as a per-cpu kernel >> > register similar to the way r13 is used for the paca. >> >> Why not use r13 instead ? We don't need to access the PACA that often >> from C code, I thought we could flip them... > > It ended up being a bit too tricky to do it that way. We can't get > directly to per-CPU data from the per-cpu data offset in exception > entry code for a number of reasons. So we end up having to load the > paca first. > > I looked at swapping r13 from paca to per-cpu offset at the same > time the stack is set up, so paca is used for early save areas then > per-cpu is used in C code. In practice it ended up getting too > tricky and fragile dealing with nested interrupts, machine checks, > etc. > > I think it's something we might slowly work towards (consolidating > back to one fixed register), but as things are it didn't work well.
Yep, agree 100%. We've talked about flipping r13 for 10 years, rather than spending another 10 this is a good way to evolve things in the right direction. cheers
