docs/nvdimm: add description of alignment requirement of device dax

For device dax (e.g., /dev/dax0.0), the NUM of 'align=NUM' option
needs to match the alignment requirement of the device dax.
It must be larger than or equal to the 'align' of device dax.
Reviewed-by: NJoao Martins <joao.m.martins@oracle.com>
Signed-off-by: NJingqi Liu <jingqi.liu@intel.com>
Message-Id: <20200429085011.63752-3-jingqi.liu@intel.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

docs/nvdimm.txt

@@ -132,6 +132,16 @@ address to the page size (getpagesize(2)) by default. However, some
 types of backends may require an alignment different than the page
 size. In that case, QEMU v2.12.0 and later provide 'align' option to
 memory-backend-file to allow users to specify the proper alignment.
+For device dax (e.g., /dev/dax0.0), this alignment needs to match the
+alignment requirement of the device dax. The NUM of 'align=NUM' option
+must be larger than or equal to the 'align' of device dax.
+We can use one of the following commands to show the 'align' of device dax.
+
+    ndctl list -X
+    daxctl list -R
+
+In order to get the proper 'align' of device dax, you need to install
+the library 'libdaxctl'.
 
 For example, device dax require the 2 MB alignment, so we can use
 following QEMU command line options to use it (/dev/dax0.0) as the

exec.c

@@ -2379,7 +2379,7 @@ RAMBlock *qemu_ram_alloc_from_fd(ram_addr_t size, MemoryRegion *mr,
     file_align = get_file_align(fd);
     if (file_align > 0 && mr && file_align > mr->align) {
         error_setg(errp, "backing store align 0x%" PRIx64
-                   " is larger than 'align' option 0x" PRIx64,
+                   " is larger than 'align' option 0x%" PRIx64,
                    file_align, mr->align);
         return NULL;
     }