x86 PAT: fix performance drop for glx, use UC minus for ioremap(), ioremap_nocache() and pci_mmap_page_range()

Use UC_MINUS for ioremap(), ioremap_nocache() instead of strong UC.
Once all the X drivers move to ioremap_wc(), we can go back to strong
UC semantics for ioremap() and ioremap_nocache().

To avoid attribute aliasing issues, pci_mmap_page_range() will also
use UC_MINUS for default non write-combining mapping request.

Next steps:
	a) change all the video drivers using ioremap() or ioremap_nocache()
	   and adding WC MTTR using mttr_add() to ioremap_wc()

	b) for strict usage, we can go back to strong uc semantics
	   for ioremap() and ioremap_nocache() after some grace period for
	   completing step-a.

	c) user level X server needs to use the appropriate method for setting
	   up WC mapping (like using resourceX_wc sysfs file instead of
	   adding MTRR for WC and using /dev/mem or resourceX under /sys)

Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com>
Signed-off-by: Venkatesh Pallipadi <venkatesh.pallipadi@intel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

1 files changed, 16 insertions, 4 deletions

diff --git a/arch/x86/mm/ioremap.c b/arch/x86/mm/ioremap.c
index 0be9f9c59aa6..6d02a36e5e49 100644
--- a/arch/x86/mm/ioremap.c
+++ b/arch/x86/mm/ioremap.c
@@ -176,11 +176,11 @@ static void __iomem *__ioremap_caller(resource_size_t phys_addr,
                 /*
                  * Do not fallback to certain memory types with certain
                  * requested type:
-                 * - request is uncached, return cannot be write-back
-                 * - request is uncached, return cannot be write-combine
+                 * - request is uc-, return cannot be write-back
+                 * - request is uc-, return cannot be write-combine
                  * - request is write-combine, return cannot be write-back
                  */
-                if ((prot_val == _PAGE_CACHE_UC &&
+                if ((prot_val == _PAGE_CACHE_UC_MINUS &&
                      (new_prot_val == _PAGE_CACHE_WB ||
                       new_prot_val == _PAGE_CACHE_WC)) ||
                     (prot_val == _PAGE_CACHE_WC &&
@@ -201,6 +201,9 @@ static void __iomem *__ioremap_caller(resource_size_t phys_addr,
         default:
                 prot = PAGE_KERNEL_NOCACHE;
                 break;
+        case _PAGE_CACHE_UC_MINUS:
+                prot = PAGE_KERNEL_UC_MINUS;
+                break;
         case _PAGE_CACHE_WC:
                 prot = PAGE_KERNEL_WC;
                 break;
@@ -255,7 +258,16 @@ static void __iomem *__ioremap_caller(resource_size_t phys_addr,
  */
 void __iomem *ioremap_nocache(resource_size_t phys_addr, unsigned long size)
 {
-        return __ioremap_caller(phys_addr, size, _PAGE_CACHE_UC,
+        /*
+         * Ideally, this should be:
+         *      pat_wc_enabled ? _PAGE_CACHE_UC : _PAGE_CACHE_UC_MINUS;
+         *
+         * Till we fix all X drivers to use ioremap_wc(), we will use
+         * UC MINUS.
+         */
+        unsigned long val = _PAGE_CACHE_UC_MINUS;
+
+        return __ioremap_caller(phys_addr, size, val,
                                 __builtin_return_address(0));
 }
 EXPORT_SYMBOL(ioremap_nocache);