intel_iommu: fix and simplify size calculation in process_device_iotl…

…b_desc()

We don't use 1ULL which is wrong during size calculation. Fix it, and
while at it, switch to use cto64() and adds a comments to make it
simpler and easier to be understood.

Reported-by: Paolo Bonzini <pbonzini@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>

hw/i386/intel_iommu.c

@@ -1485,8 +1485,16 @@ static bool vtd_process_device_iotlb_desc(IntelIOMMUState *s,
         goto done;
     }
 
+    /* According to ATS spec table 2.4:
+     * S = 0, bits 15:12 = xxxx     range size: 4K
+     * S = 1, bits 15:12 = xxx0     range size: 8K
+     * S = 1, bits 15:12 = xx01     range size: 16K
+     * S = 1, bits 15:12 = x011     range size: 32K
+     * S = 1, bits 15:12 = 0111     range size: 64K
+     * ...
+     */
     if (size) {
-        sz = 1 << (ctz64(~(addr | (VTD_PAGE_MASK_4K - 1))) + 1);
+        sz = (VTD_PAGE_SIZE * 2) << cto64(addr >> VTD_PAGE_SHIFT);
         addr &= ~(sz - 1);
     } else {
         sz = VTD_PAGE_SIZE;