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Re: [PATCH] x86: adjust default caching mode translation tables

Started byToshi Kani <toshi.kani@hp.com>
First post2015-07-22 17:30 +0200
Last post2015-07-22 18:50 +0200
Articles 5 — 2 participants

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  Re: [PATCH] x86: adjust default caching mode translation tables Toshi Kani <toshi.kani@hp.com> - 2015-07-22 17:30 +0200
    Re: [PATCH] x86: adjust default caching mode translation  tables "Jan Beulich" <JBeulich@suse.com> - 2015-07-22 17:50 +0200
      Re: [PATCH] x86: adjust default caching mode translation tables Toshi Kani <toshi.kani@hp.com> - 2015-07-22 18:10 +0200
        Re: [PATCH] x86: adjust default caching mode translation  tables "Jan Beulich" <JBeulich@suse.com> - 2015-07-22 18:40 +0200
          Re: [PATCH] x86: adjust default caching mode translation tables Toshi Kani <toshi.kani@hp.com> - 2015-07-22 18:50 +0200

#1190057 — Re: [PATCH] x86: adjust default caching mode translation tables

FromToshi Kani <toshi.kani@hp.com>
Date2015-07-22 17:30 +0200
SubjectRe: [PATCH] x86: adjust default caching mode translation tables
Message-ID<pP483-2op-23@gated-at.bofh.it>
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.  We need to use the PAT bit in order to support
both WC and WT.  The PAT errata causes the PAT bit ineffective.

Thanks,
-Toshi
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#1190074 — Re: [PATCH] x86: adjust default caching mode translation tables

From"Jan Beulich" <JBeulich@suse.com>
Date2015-07-22 17:50 +0200
SubjectRe: [PATCH] x86: adjust default caching mode translation tables
Message-ID<pP4ro-2L3-23@gated-at.bofh.it>
In reply to#1190057
>>> 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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#1190083

FromToshi Kani <toshi.kani@hp.com>
Date2015-07-22 18:10 +0200
Message-ID<pP4KK-3mX-3@gated-at.bofh.it>
In reply to#1190074
On Wed, 2015-07-22 at 09:41 -0600, Jan Beulich wrote:
> > 
> > > > 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.

'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()?

Thanks,
-Toshi
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#1190096 — Re: [PATCH] x86: adjust default caching mode translation tables

From"Jan Beulich" <JBeulich@suse.com>
Date2015-07-22 18:40 +0200
SubjectRe: [PATCH] x86: adjust default caching mode translation tables
Message-ID<pP5dL-3UB-3@gated-at.bofh.it>
In reply to#1190083
>>> On 22.07.15 at 18:06, <toshi.kani@hp.com> wrote:
> On Wed, 2015-07-22 at 09:41 -0600, Jan Beulich wrote:
>> > 
>> > > > 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.
> 
> '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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#1190104

FromToshi Kani <toshi.kani@hp.com>
Date2015-07-22 18:50 +0200
Message-ID<pP5ns-45X-7@gated-at.bofh.it>
In reply to#1190096
On Wed, 2015-07-22 at 10:36 -0600, Jan Beulich wrote:
> > 
> > > > On 22.07.15 at 18:06, <toshi.kani@hp.com> wrote:
> > On Wed, 2015-07-22 at 09:41 -0600, Jan Beulich wrote:
> > > > 
> > > > > > 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.
> > 
> > '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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