iommu/vt-d: fix SAGAW capability parsing

SAGAW is a bitmap field, with bits 1, 2 and 3 signaling support for 3, 4 and 5
level page tables respectively.  According to the Intel VT-d specification, an
IOMMU can report multiple SAGAW bits being set.

Commit 859d11b279 claims to replace the open-coded find_first_set_bit(), but
it's actually replacing an open coded implementation to find the last set bit.
The change forces the used AGAW to the lowest supported by the IOMMU instead of
the highest one between 1 and 2.

Restore the previous SAGAW parsing by using fls() instead of
find_first_set_bit(), in order to get the highest (supported) AGAW to be used.

However there's a caveat related to the value the AW context entry field must
be set to when using passthrough mode:

"When the Translation-type (TT) field indicates pass-through processing (10b),
this field must be programmed to indicate the largest AGAW value supported by
hardware." [0]

Newer Intel IOMMU implementations support 5 level page tables for the IOMMU,
and signal such support in SAGAW bit 3.

Enabling 5 level paging support (AGAW 3) is too risky at this point in the Xen
4.18 release, so instead put a bodge to unconditionally disable passthough
mode if SAGAW has any bits greater than 2 set.  Ignore bit 0; it's reserved in
current specifications, but had a meaning in the past and is unlikely to be
reused in the future.

Note the message about unhandled SAGAW bits being set is printed
unconditionally, regardless of whether passthrough mode is enabled.  This is
done in order to easily notice IOMMU implementations with not yet supported
SAGAW values.

[0] Intel VT Directed Spec Rev 4.1

Fixes: 859d11b279 ('VT-d: prune SAGAW recognition')
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Release-acked-by: Henry Wang <Henry.Wang@arm.com>

xen/drivers/passthrough/vtd/iommu.c

@@ -1327,15 +1327,25 @@ int __init iommu_alloc(struct acpi_drhd_unit *drhd)
 
     /* Calculate number of pagetable levels: 3 or 4. */
     sagaw = cap_sagaw(iommu->cap);
-    if ( sagaw & 6 )
-        agaw = find_first_set_bit(sagaw & 6);
-    if ( !agaw )
+    agaw = fls(sagaw & 6) - 1;
+    if ( agaw <= 0 )
     {
         printk(XENLOG_ERR VTDPREFIX "IOMMU: unsupported sagaw %x\n", sagaw);
         print_iommu_regs(drhd);
         rc = -ENODEV;
         goto free;
     }
+
+    if ( sagaw >> 3 )
+    {
+        printk_once(XENLOG_WARNING VTDPREFIX
+                    " Unhandled bits in SAGAW %#x%s\n",
+                    sagaw,
+                    iommu_hwdom_passthrough ? ", disabling passthrough" : "");
+
+        iommu_hwdom_passthrough = false;
+    }
+
     iommu->nr_pt_levels = agaw_to_level(agaw);
     if ( min_pt_levels > iommu->nr_pt_levels )
         min_pt_levels = iommu->nr_pt_levels;