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Re: [Qemu-discuss] Windows COM port

From: Jakob Bohm
Subject: Re: [Qemu-discuss] Windows COM port
Date: Sat, 13 Jul 2013 17:46:14 +0200
User-agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:17.0) Gecko/20130620 Thunderbird/17.0.7

(Top posting to at least keep the thread consistent)

In the traditional (pre-Vista) NT-based Windows, ISA serial ports
are detected by the real mode NTDETECT.COM, which is run from the
boot loader, I am not sure how the detection works in the new
(Vista+) boot loader, and if there is some BCDEDIT or registry
setting to enable support for "legacy" COM3 and higher.

My current best guess (looking at the URL below) is to add the
registry value

Windows Registry Editor Version 5.00


*Microsoft URL:


If this does not help, nearby pages describe how to manually enter
the port, IRQ etc. of additional ISA serial ports.

On 10-07-2013 11:18, Eric Viseur wrote:
Damn me and my bad habits of sending mails too fast. As the subject but not the content of the mail tells, the problem shows up on a Windows 7 guest. Everything works fine with a Linux guest. However I just checked and they don't appear as PCI devices on the Linux guest. Which could be the reason why I don't see them in my Windows guest...

Eric Viseur

2013/7/9 Jakob Bohm <address@hidden <mailto:address@hidden>>

    On 09-07-2013 16:08, Eric Viseur wrote:


        I'm currently setting up a virtualization infrastructure using
        Fedora 18 and KVM.  I'd like to have a total of 3 serial ports
        : the first one used for serial console, and two classic
        serial ports.
        The problem is, only two of the three serial ports show up in
        the virtual machine.  I was advised to ask the question on
        this mailing-list by the Fedora Virt Team.

        Does anyone have any insight on this problem ? Thanks in advance !

    3 Simple questions to check first:

    1. Are the serial ports emulated as PCI or ISA bus devices?

    2. If emulated as PCI, do they show up if you run lspci inside the

    3. If emulated as ISA, is the Guest kernel set up/compiled to
    allow more
    than 2 legacy serial ports and sharing of serial port interrupts,
    both kernel
    config options are needed to use more than 2 legacy ISA serial ports
    (unless you do some really oldfashioned tricks with emulated
    nonstandard IRQ assignments, kernel/module commandlines and
    the setserial program).  For non-Linux guests similar issues apply.


Jakob Bohm, CIO, Partner, WiseMo A/S.  http://www.wisemo.com
Transformervej 29, 2730 Herlev, Denmark.  Direct +45 31 13 16 10
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