x86: add gbpages switches

These new controls toggle experimental support for a new CPU feature,
the straightforward extension of largepages from the pmd level to the
pud level, which allows 1GB (kernel) TLBs instead of 2MB TLBs.

Turn it off by default, as this code has not been tested well enough yet.

Use the CONFIG_DIRECT_GBPAGES=y .config option or gbpages on the
boot line can be used to enable it. If enabled in the .config then
nogbpages boot option disables it.
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

Documentation/x86_64/boot-options.txt

@@ -307,3 +307,8 @@ Debugging
 			stuck (default)
 
 Miscellaneous
+
+	nogbpages
+		Do not use GB pages for kernel direct mappings.
+	gbpages
+		Use GB pages for kernel direct mappings.

arch/x86/Kconfig.debug

@@ -76,6 +76,18 @@ config DEBUG_RODATA
 	  data. This is recommended so that we can catch kernel bugs sooner.
 	  If in doubt, say "Y".
 
+config DIRECT_GBPAGES
+	bool "Enable gbpages-mapped kernel pagetables"
+	depends on DEBUG_KERNEL && EXPERIMENTAL && X86_64
+	help
+	  Enable gigabyte pages support (if the CPU supports it). This can
+	  improve the kernel's performance a tiny bit by reducing TLB
+	  pressure.
+
+	  This is experimental code.
+
+	  If in doubt, say "N".
+
 config DEBUG_RODATA_TEST
 	bool "Testcase for the DEBUG_RODATA feature"
 	depends on DEBUG_RODATA

arch/x86/mm/init_64.c

@@ -54,6 +54,26 @@ static unsigned long dma_reserve __initdata;
 
 DEFINE_PER_CPU(struct mmu_gather, mmu_gathers);
 
+int direct_gbpages __meminitdata
+#ifdef CONFIG_DIRECT_GBPAGES
+				= 1
+#endif
+;
+
+static int __init parse_direct_gbpages_off(char *arg)
+{
+	direct_gbpages = 0;
+	return 0;
+}
+early_param("nogbpages", parse_direct_gbpages_off);
+
+static int __init parse_direct_gbpages_on(char *arg)
+{
+	direct_gbpages = 1;
+	return 0;
+}
+early_param("gbpages", parse_direct_gbpages_on);
+
 /*
  * NOTE: pagetable_init alloc all the fixmap pagetables contiguous on the
  * physical space so we can cache the place of the first one and move

include/asm-x86/pgtable_64.h

@@ -239,6 +239,8 @@ static inline int pud_large(pud_t pte)
 
 #define update_mmu_cache(vma,address,pte) do { } while (0)
 
+extern int direct_gbpages;
+
 /* Encode and de-code a swap entry */
 #define __swp_type(x)			(((x).val >> 1) & 0x3f)
 #define __swp_offset(x)			((x).val >> 8)