>>> 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.
Jan
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