Add a kludge-around to fix cd/wt bits in fb ptes on linux.

Mmap from /sys/devices/pci* on linux forces the cache-disable and
write-through bits, which turns our write-combining map into an
uncached-map, seriously impacting performance. It turns out that a bug in
mprotect allows us to fix this by disabling access to those pages and then
immediately re-enabling them.

configure.ac

@@ -44,6 +44,9 @@ AM_PROG_CC_C_O
 AC_CHECK_PROG(gen4asm, [intel-gen4asm], yes, no)
 AM_CONDITIONAL(HAVE_GEN4ASM, test x$gen4asm = xyes)
 
+AC_CHECK_HEADERS(sys/mman.h)
+AC_CHECK_FUNCS(mprotect)
+
 AH_TOP([#include "xorg-server.h"])
 
 AC_ARG_WITH(xorg-module-dir,

src/i830_driver.c

@@ -197,6 +197,9 @@ USE OR OTHER DEALINGS IN THE SOFTWARE.
 #include "i830_debug.h"
 #include "i830_bios.h"
 #include "i830_video.h"
+#if HAVE_SYS_MMAN_H && HAVE_MPROTECT
+#include <sys/mman.h>
+#endif
 
 #ifdef INTEL_XVMC
 #define _INTEL_XVMC_SERVER_
@@ -685,6 +688,13 @@ I830MapMem(ScrnInfoPtr pScrn)
    err = pci_device_map_range (device, pI830->LinearAddr, pI830->FbMapSize,
 			       PCI_DEV_MAP_FLAG_WRITABLE | PCI_DEV_MAP_FLAG_WRITE_COMBINE,
 			       (void **) &pI830->FbBase);
+    if (err)
+	return FALSE;
+    /* KLUDGE ALERT -- rewrite the PTEs to turn off the CD and WT bits */
+#if HAVE_MPROTECT
+    mprotect (pI830->FbBase, pI830->FbMapSize, PROT_NONE);
+    mprotect (pI830->FbBase, pI830->FbMapSize, PROT_READ|PROT_WRITE);
+#endif
 #else
    pI830->FbBase = xf86MapPciMem(pScrn->scrnIndex, VIDMEM_FRAMEBUFFER,
 				 pI830->PciTag,