Xen/cdrom: Ubuntu 16.04 VM read the content from CD-ROM abnormally

[Ken <jinjian@huawei.com>]

Hi all,

I run the Ubuntu 16.04 server (2 vcpu/2G, Linux 4.4.0) on the Xen-4.1.2, 
and installed gcc through the CDROM used by 16.04 iso file, when I 
installed gcc that depends deb packages to decompress failed. But 
uploaded the ISO files into the VM are mounted by loop or used as CDROM 
for other VM (Fedora 24) transport these abnormal files to 16.04 are 
available. So the 16.04 ISO file should be correct.

Then I went to try to fix this problem, the steps are as follows:
First of all, I was worried because the Hypervisor version is too old to 
cause this problems, so I upgraded to the Xen upstream and found that 
there are the problem still.

Then, I went to attempted to upgrade the VM kernel, by dichotomy to find 
the smallest available kernel, but found that the kernel patch has 
nothing to do the cdrom or isofs driver, Linux kernel committed history 
has nothing to do these, combined with the Fedora within Linux 4.4.0 
have no problem, so I inferred the kernel had no this defective.

The above comparison can not troubleshoot the problem, so I analyzed the 
Linux CD-ROM device loading process to confirm that the problem is 
encountered before the mount. and I found the following three strange 
phenomenon:
1. Using Ubuntu 14.04.5 udevadm, re-packaged the init-ramfs of 16.04, 
reboot the VM, read the content of CD-ROM successfully, but compile the 
system-204 from the 14.04.5 and installed on 16.04, the problem still 
can not be resolved, some abnormal logs as blow:
    random: udevadm: uninitialized urandom read (16 bytes read, 28 bits 
of entropy available)

2. found that the VM registers dmi failed at startup, so I removed the 
dmi driver, reboot the VM, read the content of CD-ROM successfully, and 
from the kernel dmi_scan_machine code, it should not affect to use the 
CD-ROM, The failed logs as blow:
    ioremap error for 0xfc001000-0xfc002000, requested 0x2, got 0x0
    dmi: Firmware registration failed.

3. Deploy the VM within 8 vcpus, 8G memory, read the content of CD-ROM 
successfully.

Therefore, I inferred this anomaly that has several problems, but can 
not focus it, there are other people have encountered this same anomaly? 
Do anyone have any suggests to
debug it next step?

Thanks.