xen/amd/iommu: Don't swap the IVMD read and write permissions

Per the IVMD Flags definition, bit 1 is IR and bit 2 is IW, matching
ACPI_IVMD_READ and ACPI_IVMD_WRITE.  parse_ivmd_block() takes iw from
the former and ir from the latter, so a unity range firmware declares
read-only is mapped write-only, and vice versa.

Ranges with both permissions, including Xen's own, are unaffected.

Fixes: e941abd7ca ("ACPI: eliminate duplicate IVRS definitions")
Signed-off-by: Stefano Stabellini <stefano.stabellini@amd.com>
Reviewed-by: Teddy Astie <teddy.astie@vates.tech>
Acked-by: Roger Pau Monné <roger@xenproject.org>

xen/drivers/passthrough/amd/iommu_acpi.c

@@ -414,8 +414,8 @@ static int __init parse_ivmd_block(const struct acpi_ivrs_memory *ivmd_block)
         exclusion = true;
     else if ( ivmd_block->header.flags & ACPI_IVMD_UNITY )
     {
-        iw = ivmd_block->header.flags & ACPI_IVMD_READ;
+        iw = ivmd_block->header.flags & ACPI_IVMD_WRITE;
-        ir = ivmd_block->header.flags & ACPI_IVMD_WRITE;
+        ir = ivmd_block->header.flags & ACPI_IVMD_READ;
     }
     else
     {