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Re: [Xen-users] Serial port access in HVM (fully virtualized) DomU

To: "Hans Rakers" <hans@xxxxxxxx>
Subject: Re: [Xen-users] Serial port access in HVM (fully virtualized) DomU
From: "Trolle Selander" <trolle.selander@xxxxxxxxx>
Date: Tue, 4 Dec 2007 12:41:41 +0100
Cc: xen-users@xxxxxxxxxxxxxxxxxxx
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This is a HVM DomU, so you need to use an emulated serial port backed by the physical port. There is no support for assigning physical serial port resources to "give" the serial port directly to the HVM DomU. Theoretically, this should be fairly easily doable even on IOMMU-less systems, but as far as I know. there's no code to do this yet for HVMs. Serial ports on a separate PCI card should be possible to using the PCI passthrough feature in recent versions of xen.

However, the emulated device may work fine for you, with much less hassle. Just make sure that your serial ports _are_ available to Dom0 (that is, remove the blacklist entries), and then add serial=/dev/ttyS0 to your HVM domain config. There are some caveats with serial emulation currently - I'm working on improving this particular code right now, so if you run into any strangeness, I'll probably know what's going on and be able to help. :)

On Dec 4, 2007 12:24 PM, Hans Rakers <hans@xxxxxxxx> wrote:

Nothing, as i already stated. I'm pretty sure Dom0 doesn't load anything
serial-ish, and i've blacklisted the 8250/8250_pnp/serial_core modules
on Dom0.

Grub.conf entry of Dom0 looks like this:

title CentOS (2.6.18-53.1.4.el5xen)
        root (hd0,0)
        kernel /xen.gz-2.6.18-53.1.4.el5 console=vga
        module /vmlinuz-2.6.18-53.1.4.el5xen
root=/dev/VolGroup00/LogVol01 ro xencons=off
        module /initrd-2.6.18-53.1.4.el5xen.img

Kind regards,


Sadique Puthen wrote:
> What is the output of "dmesg | grep ttyS" in dom0?
> --Sadique
> Hans Rakers wrote:
>> Hello xen-users,
>> Last days i've been pulling my hair on getting ttyS0 (COM1) to work in
>> a HVM DomU. I want to use it for an external faxmodem for use with
>> Hylafax. I've Googled myself crazy and have seen just about every page
>> on this subject :)
>> My system is CentOS 5.1 with xen-3.0.3-41.el5. I've added xencons=off
>> to grub.conf of DomU and i've added the following to
>> /etc/modprobe.d/blacklist:
>> blacklist 8250
>> blacklist 8250_pnp
>> blacklist serial_core
>> I can confirm that Dom0 does not claim the serial ports (no mention of
>> it during boot, no modules loaded, xencons off)
>> My DomU also runs CentOS 5.1. I've added the following to its vm config:
>> irq = [3, 4]
>> ioports = ["3f8-3ff", "2f8-2ff"]
>> Syslog of the DomU mentions the loading of the 8250 module, but no
>> resulting ports:
>> Serial: 8250/16550 driver $Revision: 1.90 $ 4 ports, IRQ sharing enabled
>> /proc/ioports of the DomU also doesn't mention the mapped ioports:
>> [...snip snip...]
>> 01f0-01f7 : ide0
>> 03c0-03df : vga+
>> 03f2-03f5 : floppy
>> 03f6-03f6 : ide0
>> 03f7-03f7 : floppy DIR
>> 0cf8-0cff : PCI conf1
>> [...snip snip...]
>> Is this at all possible, or am i overlooking something stupid? Do
>> 'ioports' and 'irq' work with fully virtualized VM's? I've read
>> something earlier on this list at
>> http://lists.xensource.com/archives/html/xen-users/2007-08/msg00838.html
>> which states that it is impossible for HVMs to have direct access to
>> hardware, which is why i ask, before spending another day on this :)
>> Thanks in advance,
>> Hans Rakers
>> _______________________________________________
>> Xen-users mailing list
>> Xen-users@xxxxxxxxxxxxxxxxxxx
>> http://lists.xensource.com/xen-users
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