x86/PCI/PAT: return EINVAL for pci mmap WC request for !pat_enabled

Thomas Schlichter reported:
> X.org uses libpciaccess which tries to mmap with write combining enabled via
> /sys/bus/pci/devices/*/resource0_wc. Currently, when PAT is not enabled, the
> kernel does fall back to uncached mmap. Then libpciaccess thinks it succeeded
> mapping with write combining enabled and does not set up suited MTRR entries.
> ;-(

Instead of silently mapping pci mmap region as UC minus in the case
of !pat_enabled and wc request, we can return error. Eric Anholt mentioned
that caller (like X) typically follows up with UC minus pci mmap request and
if there is a free mtrr slot, caller will manage adding WC mtrr.

Jesse Barnes says:
> Older versions of libpciaccess will behave better if we do it that way
> (iirc it only allocates an MTRR if the resource_wc file doesn't exist or
> fails to get mapped).

Reported-by: Thomas Schlichter <thomas.schlichter@web.de>
Signed-off-by: Thomas Schlichter <thomas.schlichter@web.de>
Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com>
Acked-by: Eric Anholt <eric@anholt.net>
Acked-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>

1 files changed, 9 insertions, 0 deletions

diff --git a/arch/x86/pci/i386.c b/arch/x86/pci/i386.c
index a70a85de5e84..52e656f17781 100644
--- a/arch/x86/pci/i386.c
+++ b/arch/x86/pci/i386.c
@@ -280,6 +280,15 @@ int pci_mmap_page_range(struct pci_dev *dev, struct vm_area_struct *vma,
                 return -EINVAL;
 
         prot = pgprot_val(vma->vm_page_prot);
+
+        /*
+         * Return error if pat is not enabled and write_combine is requested.
+         * Caller can followup with UC MINUS request and add a WC mtrr if there
+         * is a free mtrr slot.
+         */
+        if (!pat_enabled && write_combine)
+                return -EINVAL;
+
         if (pat_enabled && write_combine)
                 prot |= _PAGE_CACHE_WC;
         else if (pat_enabled || boot_cpu_data.x86 > 3)