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

[Juergen Gross <jgross@xxxxxxxx>]

On 01/22/2015 08:11 AM, Steven Noonan wrote:
> On Mon, Nov 3, 2014 at 5:01 AM, Juergen Gross <jgross@xxxxxxxx> 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@xxxxxxxxxxxxx>
> > Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
> > Reviewed-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> > ---
...
> > 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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