Re: What is the purpose setting console=hvc0 in the dom0 grub config?

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On Wed, May 17, 2017 at 2:39 AM, George Dunlap <dunlapg@xxxxxxxxx> wrote:
On Wed, May 17, 2017 at 4:26 AM, Jerry <jerryubi@xxxxxxxxx> wrote:
> I always disable "rhgb quiet" on a fresh install because I don't like boot
> messages being hidden from me, and now this other thing does it.  I like
> details, I need the details, don't hide them from me.

I feel the same way about 'rhgb quiet'.  :-)

The 'console=hvc0' setting doesn't hide them from you, it just sends
them somewhere you're not looking.

I'm looking at the system's console during installation. Sending it somewhere else is hiding it from me.
On bare metal, the console output can typically go two places:
1. The screen
2. A serial port

For server applications serial has several advantages over the screen:
* You can capture the output to more easily report bugs
* If you're capturing it you can keep things that would have scrolled
off-screen, or been erased due to a reboot
* In a datacenter it's faster, more convenient, and cheaper than an
IP-based KVM switch

I get what these things are, but not what hvc0 is doing.

This system has built-in IPMI, the installation was done remotely using it.
Xen has the same two options above; but when Linux is running as a
dom0 under Xen, there are three places to put it:
1. The screen
2. A serial line
3. Send it to Xen to put wherever Xen is putting it

#1 is easy, but #2 is tricky because Xen is likely to be already using
the serial port you want to use.

"console=hvc0" is #3.

What's your Xen command-line look like?  The default should be
"console=com1,tty", so Xen's output should show up both places (and so
should Linux's if it's set to console=hvc0).

This is what's defined in /etc/default/grub following the install of the Xen:

GRUB_CMDLINE_XEN_DEFAULT="dom0_mem=1024M,max:1024M cpuinfo com1=115200,8n1 console=com1,tty loglvl=all guest_loglvl=all"
GRUB_CMDLINE_LINUX_XEN_REPLACE_DEFAULT="console=hvc0 earlyprintk=xen nomodeset"

I didn't set these myself, this is what the xen package (or one of its dependencies) is doing.

I'm still not clear on why hvc0 is needed, or why it's being set, but what I do know for sure is it was causing the boot messages to be suppressed.

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