On Thu, 2014-09-04 at 17:51 -0700, Andy Lutomirski wrote: > On Thu, Sep 4, 2014 at 5:29 PM, Toshi Kani <toshi.kani@xxxxxx> wrote: > > On Thu, 2014-09-04 at 16:34 -0700, Andy Lutomirski wrote: > >> On Thu, Sep 4, 2014 at 4:19 PM, Henrique de Moraes Holschuh > >> <hmh@xxxxxxxxxx> wrote: > >> > On Thu, 04 Sep 2014, H. Peter Anvin wrote: > >> >> On 09/04/2014 01:11 PM, Henrique de Moraes Holschuh wrote: > >> >> > I am worried of uncharted territory, here. I'd actually advocate for not > >> >> > enabling the upper four PAT entries on IA-32 at all, unless Windows 9X / XP > >> >> > is using them as well. Is this a real concern, or am I being overly > >> >> > cautious? > >> >> > >> >> It is extremely unlikely that we'd have PAT issues in 32-bit mode and > >> >> not in 64-bit mode on the same CPU. > >> > > >> > Sure, but is it really a good idea to enable this on the *old* non-64-bit > >> > capable processors (note: I don't mean x86-64 processors operating in 32-bit > >> > mode) ? > >> > > >> >> As far as I know, the current blacklist rule is very conservative due to > >> >> lack of testing more than anything else. > >> > > >> > I was told that much in 2009 when I asked why cpuid 0x6d8 was blacklisted > >> > from using PAT :-) > >> > >> At the very least, anyone who plugs an NV-DIMM into a 32-bit machine > >> is nuts, and not just because I'd be somewhat amazed if it even > >> physically fits into the slot. :) > > > > According to the spec, the upper four entries bug was fixed in Pentium 4 > > model 0x1. So, the remaining Intel 32-bit processors that may enable > > the upper four entries are Pentium 4 model 0x1-4. Should we disable it > > for all Pentium 4 models? > > Assuming that this is Pentium 4 erratum N46, then there may be another > option: use slot 7 instead of slot 4 for WT. Then, even if somehow > the blacklist screws up, the worst that happens is that a WT page gets > interpreted as UC. I suppose this could cause aliasing issues, but > can't cause problems for people who don't use the high entries in the > first place. That's a fine idea, but as Ingo also suggested, I am going to disable this feature on all Pentium 4 models. That should give us a safety margin. Using slot 4 has a benefit that it keeps the PAT setup consistent with Xen. Thanks, -Toshi -- 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=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>