iommu: fix Intel IOMMU write-buffer flushing

This is the cause of the DMA faults and disk corruption that people have
been seeing. Some chipsets neglect to report the RWBF "capability" --
the flag which says that we need to flush the chipset write-buffer when
changing the DMA page tables, to ensure that the change is visible to
the IOMMU.

Override that bit on the affected chipsets, and everything is happy
again.

Thanks to Chris and Bhavesh and others for helping to debug.

Should resolve:

  https://bugzilla.redhat.com/show_bug.cgi?id=479996
  http://bugzilla.kernel.org/show_bug.cgi?id=12578

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Tested-and-acked-by: Chris Wright <chrisw@sous-sol.org>
Reviewed-by: Bhavesh Davda <bhavesh@vmware.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

drivers/pci/intel-iommu.c

@@ -61,6 +61,8 @@
 /* global iommu list, set NULL for ignored DMAR units */
 static struct intel_iommu **g_iommus;
 
+static int rwbf_quirk;
+
 /*
  * 0: Present
  * 1-11: Reserved
@@ -785,7 +787,7 @@ static void iommu_flush_write_buffer(struct intel_iommu *iommu)
 	u32 val;
 	unsigned long flag;
 
-	if (!cap_rwbf(iommu->cap))
+	if (!rwbf_quirk && !cap_rwbf(iommu->cap))
 		return;
 	val = iommu->gcmd | DMA_GCMD_WBF;
 
@@ -3137,3 +3139,15 @@ static struct iommu_ops intel_iommu_ops = {
 	.unmap		= intel_iommu_unmap_range,
 	.iova_to_phys	= intel_iommu_iova_to_phys,
 };
+
+static void __devinit quirk_iommu_rwbf(struct pci_dev *dev)
+{
+	/*
+	 * Mobile 4 Series Chipset neglects to set RWBF capability,
+	 * but needs it:
+	 */
+	printk(KERN_INFO "DMAR: Forcing write-buffer flush capability\n");
+	rwbf_quirk = 1;
+}
+
+DECLARE_PCI_FIXUP_HEADER(PCI_VENDOR_ID_INTEL, 0x2a40, quirk_iommu_rwbf);