On Thu, 2012-06-28 at 17:09 +1000, Benjamin Herrenschmidt wrote: > On Thu, 2012-06-28 at 01:01 +0200, Peter Zijlstra wrote: > > On Wed, 2012-06-27 at 15:23 -0700, Linus Torvalds wrote: > > > > > Plus it really isn't about hardware page table walkers at all. It's > > > more about the possibility of speculative TLB fils, it has nothing to > > > do with *how* they are done. Sure, it's likely that a software > > > pagetable walker wouldn't be something that gets called speculatively, > > > but it's not out of the question. > > > > > Hmm, I would call gup_fast() as speculative as we can get in software. > > It does a lock-less walk of the page-tables. That's what the RCU free'd > > page-table stuff is for to begin with. > > Strictly speaking it's not :-) To *begin with* (as in the origin of that > code) it comes from powerpc hash table code which walks the linux page > tables locklessly :-) It then came in handy with gup_fast :-) Ah, ok my bad. > > > IOW, if Sparc/PPC really want to guarantee that they never fill TLB > > > entries speculatively, and that if we are in a kernel thread they will > > > *never* fill the TLB with anything else, then make them enable > > > CONFIG_STRICT_TLB_FILL or something in their architecture Kconfig > > > files. > > > > Since we've dealt with the speculative software side by using RCU-ish > > stuff, the only thing that's left is hardware, now neither sparc64 nor > > ppc actually know about the linux page-tables from what I understood, > > they only look at their hash-table thing. > > Some embedded ppc's know about the lowest level (SW loaded PMD) but > that's not an issue here. We flush these special TLB entries > specifically and synchronously in __pte_free_tlb(). OK, I missed that.. is that arch/powerpc/mm/tlb_nohash.c:tlb_flush_pgtable() ? > > So even if the hardware did do speculative tlb fills, it would do them > > from the hash-table, but that's already cleared out. > > Right, Phew at least I got the important thing right ;-) -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href