Re: [RFC/PATCH 2/2] powerpc: 44x doesn't need G set everywhere

[Josh Boyer <jwboyer@linux.vnet.ibm.com>]

On Thu, 11 Dec 2008 07:03:57 +1100
Benjamin Herrenschmidt <benh@kernel.crashing.org> wrote:

> 
> > > We still leave G=1 on the linear mapping for now, we need to
> > > stop over-mapping RAM to be able to remove it.
> > 
> > Hm.  Over-mapping it has the nice advantage that we use as few pinned
> > TLB entries as possible.  For 440x6 cores with more than 256 MiB of
> > DRAM, you could theoretically use a single 1GiB TLB entry to map all
> > kernel DRAM.
> 
> Ok well, there are several issues here.. see below
> 
> > Do you think the trade-offs of allowing speculative accesses are worth
> > the increased TLB pressure?  Large base pages will help with that in
> > some workloads, but others are still going to be TLB constrained.
> > 
> > I know, I'm probably paranoid.  But changing things like this around
> > without some kind of benchmark data or testcase to make sure we aren't
> > making it worse gives me the heebee-geebees.
> 
> Yup, which is why I'm not changing it yet :-) My initial thinking was
> along the lines of: We can use up to 4 bolted TLB entries, that will
> cover most classic memory configurations such as 256, 512 etc.... and
> leave what doesn't fit to highmem.
> 
> However that fails miserably with 128M which is quite common.
> 
> Then I thought we could overmap and use G for things that don't quite
> fit and remove G when we know we can do an exact mapping...
> 
> Then I though .. heh, first we know there is no speculative or
> prefetched data access on 440. We also know that speculative /
> prefetched instruction access is busted and must be disabled.
> 
> Thus can't we just both overmap and not have G ?
> 
> Needs testing of course :-) I'm waiting for an answer from the chip guys
> here.

Heh.  I like it.

> G=1 has some other impacts, such as preventing write combining I think,
> re-ordering, and a few other things.

Yeah.

Overall, I'm OK with changing things as long as we can sort of prove we
aren't making it worse.

josh