Messages in this thread | | | Date | Wed, 22 Jul 2015 09:41:31 -0600 | From | "Jan Beulich" <> | Subject | Re: [PATCH] x86: adjust default caching mode translation tables |
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>>> On 22.07.15 at 17:23, <toshi.kani@hp.com> wrote: > On Wed, 2015-07-22 at 09:17 -0600, Jan Beulich wrote: >> > >> > > > On 22.07.15 at 00:29, <toshi.kani@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.
Jan
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