linux-kernel - Re: [PATCH V6 01/18] x86: Make page cache mode a real type

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Date:	Thu, 22 Jan 2015 11:15:52 +0100
From:	Juergen Gross <jgross@...e.com>
To:	Steven Noonan <steven@...inklabs.net>
CC:	"H. Peter Anvin" <hpa@...or.com>, Linux-X86 <x86@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, stefan.bader@...onical.com,
	Linux Kernel mailing List <linux-kernel@...r.kernel.org>,
	xen-devel@...ts.xensource.com,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	ville.syrjala@...ux.intel.com,
	David Vrabel <david.vrabel@...rix.com>,
	Jan Beulich <jbeulich@...e.com>, toshi.kani@...com,
	plagnioj@...osoft.com, tomi.valkeinen@...com, bhelgaas@...gle.com
Subject: Re: [PATCH V6 01/18] x86: Make page cache mode a real type

On 01/22/2015 08:11 AM, Steven Noonan wrote:
> On Mon, Nov 3, 2014 at 5:01 AM, Juergen Gross <jgross@...e.com> wrote:
>> At the moment there are a lot of places that handle setting or getting
>> the page cache mode by treating the pgprot bits equal to the cache mode.
>> This is only true because there are a lot of assumptions about the setup
>> of the PAT MSR. Otherwise the cache type needs to get translated into
>> pgprot bits and vice versa.
>>
>> This patch tries to prepare for that by introducing a separate type
>> for the cache mode and adding functions to translate between those and
>> pgprot values.
>>
>> To avoid too much performance penalty the translation between cache mode
>> and pgprot values is done via tables which contain the relevant
>> information.  Write-back cache mode is hard-wired to be 0, all other
>> modes are configurable via those tables. For large pages there are
>> translation functions as the PAT bit is located at different positions
>> in the ptes of 4k and large pages.
>>
>> Based-on-patch-by: Stefan Bader <stefan.bader@...onical.com>
>> Signed-off-by: Juergen Gross <jgross@...e.com>
>> Reviewed-by: Thomas Gleixner <tglx@...utronix.de>
>> ---
...
>> diff --git a/arch/x86/mm/init.c b/arch/x86/mm/init.c
>> index 66dba36..a9776ba 100644
>> --- a/arch/x86/mm/init.c
>> +++ b/arch/x86/mm/init.c
>> @@ -27,6 +27,35 @@
>>
>>   #include "mm_internal.h"
>>
>> +/*
>> + * Tables translating between page_cache_type_t and pte encoding.
>> + * Minimal supported modes are defined statically, modified if more supported
>> + * cache modes are available.
>> + * Index into __cachemode2pte_tbl is the cachemode.
>> + * Index into __pte2cachemode_tbl are the caching attribute bits of the pte
>> + * (_PAGE_PWT, _PAGE_PCD, _PAGE_PAT) at index bit positions 0, 1, 2.
>> + */
>> +uint16_t __cachemode2pte_tbl[_PAGE_CACHE_MODE_NUM] = {
>> +       [_PAGE_CACHE_MODE_WB]           = 0,
>> +       [_PAGE_CACHE_MODE_WC]           = _PAGE_PWT,
>> +       [_PAGE_CACHE_MODE_UC_MINUS]     = _PAGE_PCD,
>> +       [_PAGE_CACHE_MODE_UC]           = _PAGE_PCD | _PAGE_PWT,
>> +       [_PAGE_CACHE_MODE_WT]           = _PAGE_PCD,
>> +       [_PAGE_CACHE_MODE_WP]           = _PAGE_PCD,
>> +};
>> +EXPORT_SYMBOL_GPL(__cachemode2pte_tbl);
>> +uint8_t __pte2cachemode_tbl[8] = {
>> +       [__pte2cm_idx(0)] = _PAGE_CACHE_MODE_WB,
>> +       [__pte2cm_idx(_PAGE_PWT)] = _PAGE_CACHE_MODE_WC,
>> +       [__pte2cm_idx(_PAGE_PCD)] = _PAGE_CACHE_MODE_UC_MINUS,
>> +       [__pte2cm_idx(_PAGE_PWT | _PAGE_PCD)] = _PAGE_CACHE_MODE_UC,
>> +       [__pte2cm_idx(_PAGE_PAT)] = _PAGE_CACHE_MODE_WB,
>> +       [__pte2cm_idx(_PAGE_PWT | _PAGE_PAT)] = _PAGE_CACHE_MODE_WC,
>> +       [__pte2cm_idx(_PAGE_PCD | _PAGE_PAT)] = _PAGE_CACHE_MODE_UC_MINUS,
>> +       [__pte2cm_idx(_PAGE_PWT | _PAGE_PCD | _PAGE_PAT)] = _PAGE_CACHE_MODE_UC,
>> +};
>> +EXPORT_SYMBOL_GPL(__pte2cachemode_tbl);
>> +
>
> I notice these two symbols are exported GPL-only. This breaks builds
> of several out-of-tree non-GPL modules such as the NVIDIA driver, and
> VMware modules, etc. What is the appropriate code path for proprietary
> modules to use when setting page cache mode flags? Alternatively, is
> it possible for these EXPORT_SYMBOL_GPLs to be changed to
> EXPORT_SYMBOL?

I don't mind you sending a patch to change this. I won't object such a
patch. OTOH this is more kind of a political question and I don't want
to spend my time on arguing.

Juergen

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