On Wed, 2015-07-22 at 10:36 -0600, Jan Beulich wrote:
> >
> > > > On 22.07.15 at 18:06, <toshi.k...@hp.com> wrote:
> > On Wed, 2015-07-22 at 09:41 -0600, Jan Beulich wrote:
> > > >
> > > > > > On 22.07.15 at 17:23, <toshi.k...@hp.com> wrote:
> > > > On Wed, 2015-07-22 at 09:17 -0600, Jan Beulich wrote:
> > > > > >
> > > > > > > > On 22.07.15 at 00:29, <toshi.k...@hp.com> wrote:
> > > > > > On Mon, 2015-07-20 at 08:46 +0100, Jan Beulich wrote:
> > > > > > > Make WT really mean WT (rather than UC).
> > > > > > >
> > > > > > > I can't see why commit 9cd25aac1f ("x86/mm/pat: Emulate PAT
> > > > > > > when
> > > > > > > it
> > > > > > > is
> > > > > > > disabled") didn't make this match its changes to pat_init().
> > > > > >
> > > > > > No, the default values need to be set to the fallback types,
> > > > > > i.e.
> > > > > > minimal
> > > > > > supported mode. For WC and WT, UC is the fallback type.
> > > > >
> > > > > But why would that be?
> > > > >
> > > > > > When PAT is disabled, pat_init() does update the tables below
> > > > > > to
> > > > > > enable
> > > > > > WT
> > > > > > per the default BIOS setup. However, when PAT is enabled, but
> > > > > > CPU
> > > > > > has
> > > > > > PAT
> > > > > > -errata, WT falls back to UC per the default values.
> > > > >
> > > > > PAT related errata I'm aware of are related to either page size
> > > > > or
> > > > > the number of bits used to index into the PAT MSR, but never to
> > > > > a particular memory type. Are you saying there are errata which
> > > > > make use of WT or WC impossible altogether? Otherwise I would
> > > > > have thought (even more so in the absence of any comment
> > > > > saying otherwise - "minimal supported modes" doesn't really say
> > > > > on what basis the set is the minimal one) that the mode systems
> > > > > come up in (compatible with pre-PAT) ought to be what the tables
> > > > > express.
> > > >
> > > > Please take a look at the comments in pat_init(). WT uses slot 7
> > > > (not
> > > > slot
> > > > 1) in the regular case.
> > >
> > > But that is an adjustment Linux makes to the default the system
> > > comes up in. And again - in my opinion the pre-initialized table
> > > values should reflect the mode the system comes up in (i.e.
> > > correct prior to execution reaching pat_init()), and be updated
> > > (which as it seems happens in all three possible cases) once the
> > > MSR gets fiddled with.
> >
> > 'enum page_cache_mode' thru __cachemode2pte_tbl[] is a high-level
> > abstraction that can be used after pat_init(). For early boot-time,
> > __early_ioremap() takes pgprot_t directly.
> >
> > Do you have a need to use __cachemode2pte_tbl[] before pat_init()?
>
> No. I just noticed the (apparent?) inconsistency.
I will update the comment of __cachemode2pte_tbl[] to avoid such confusion.
Thanks,
-Toshi
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