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Re: [Xen-devel] [PATCH RFC 1/3] x86: Make page cache mode a real type

On 08/22/2014 12:09 AM, Toshi Kani wrote:
On Tue, 2014-08-19 at 15:25 +0200, jgross@xxxxxxxx wrote:
From: Juergen Gross <jgross@xxxxxxxx>

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 seperate type
for the cache mode and adding functions to translate between those and pgprot

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.

One more comment below..

diff --git a/arch/x86/include/asm/cacheflush.h 
-static inline void set_page_memtype(struct page *pg, unsigned long memtype)
+static inline void set_page_memtype(struct page *pg,
+                                   enum page_cache_mode memtype)
        unsigned long memtype_flags = _PGMT_DEFAULT;
        unsigned long old_flags;
        unsigned long new_flags;

        switch (memtype) {
-       case _PAGE_CACHE_WC:
+       case _PAGE_CACHE_MODE_WC:
                memtype_flags = _PGMT_WC;
-       case _PAGE_CACHE_UC_MINUS:
                memtype_flags = _PGMT_UC_MINUS;
-       case _PAGE_CACHE_WB:
+       case _PAGE_CACHE_MODE_WB:
+       default:
                memtype_flags = _PGMT_WB;

Adding the "default" case handled as _PGMT_WB is not correct here.
free_ram_pages_type() calls set_page_memtype() with -1, which needs to
be set to _PGMT_DEFAULT.

It says so in the comment above. I'll correct it, thanks.


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