IOMMU/ATS: fix maximum queue depth calculation

The capabilities register field is a 5-bit value, and the 5 bits all
being zero actually means 32 entries.

Under the assumption that amd_iommu_flush_iotlb() really just tried
to correct for the miscalculation above when adding 32 to the value,
that adjustment is also being removed.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/drivers/passthrough/amd/iommu_cmd.c
+++ b/xen/drivers/passthrough/amd/iommu_cmd.c
@@ -321,7 +321,7 @@ void amd_iommu_flush_iotlb(struct pci_de
 
     req_id = get_dma_requestor_id(iommu->seg, bdf);
     queueid = req_id;
-    maxpend = (ats_pdev->ats_queue_depth + 32) & 0xff;
+    maxpend = ats_pdev->ats_queue_depth & 0xff;
 
     /* send INVALIDATE_IOTLB_PAGES command */
     spin_lock_irqsave(&iommu->lock, flags);
--- a/xen/drivers/passthrough/ats.h
+++ b/xen/drivers/passthrough/ats.h
@@ -28,7 +28,7 @@ struct pci_ats_dev {
 
 #define ATS_REG_CAP    4
 #define ATS_REG_CTL    6
-#define ATS_QUEUE_DEPTH_MASK     0xF
+#define ATS_QUEUE_DEPTH_MASK     0x1f
 #define ATS_ENABLE               (1<<15)
 
 extern struct list_head ats_devices;
--- a/xen/drivers/passthrough/x86/ats.c
+++ b/xen/drivers/passthrough/x86/ats.c
@@ -94,6 +94,8 @@ int enable_ats_device(int seg, int bus, 
         value = pci_conf_read16(seg, bus, PCI_SLOT(devfn),
                                 PCI_FUNC(devfn), pos + ATS_REG_CAP);
         pdev->ats_queue_depth = value & ATS_QUEUE_DEPTH_MASK;
+        if ( !pdev->ats_queue_depth )
+            pdev->ats_queue_depth = ATS_QUEUE_DEPTH_MASK + 1;
         list_add(&pdev->list, &ats_devices);
     }