CentOS virt - May 2017 - What is the purpose setting console=hvc0 in the dom0 grub config?

Howdy,

I recently went through a frustrating experience trying to get Xen 4
running on a CentOS 7 system.  After a fresh install, fully updating the
system, rebooting, then trying to install Xen4CentOS it would fail to boot
into the 4.9 kernel, sitting there with a blinking cursor indefinitely.

I thought it was a failure with grub, but it turns out this was set in the
grub config:

GRUB_CMDLINE_LINUX_XEN_REPLACE_DEFAULT="console=hvc0 ..."

So the kernel was loading successfully, but it was failing boot properly
(hanging because it couldn't find the / file system, but that's a
different
story). Boot messages were not being displayed on my console because of the
console=hvc0 setting, hampering the troubleshooting process.

What is the purpose of setting console=hvc0?  I removed it so I could
trouble shoot this problem, but I didn't re-add it and the system booted
fine, Xen is working.  Is there something that depends on this?

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.

Thanks,
Jerry
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On Wed, May 17, 2017 at 4:26 AM, Jerry <jerryubi at gmail.com>
wrote:
> Howdy,
>
> I recently went through a frustrating experience trying to get Xen 4
running
> on a CentOS 7 system.  After a fresh install, fully updating the system,
> rebooting, then trying to install Xen4CentOS it would fail to boot into the
> 4.9 kernel, sitting there with a blinking cursor indefinitely.
>
> I thought it was a failure with grub, but it turns out this was set in the
> grub config:
>
> GRUB_CMDLINE_LINUX_XEN_REPLACE_DEFAULT="console=hvc0 ..."
>
> So the kernel was loading successfully, but it was failing boot properly
> (hanging because it couldn't find the / file system, but that's a
different
> story). Boot messages were not being displayed on my console because of the
> console=hvc0 setting, hampering the troubleshooting process.
>
> What is the purpose of setting console=hvc0?  I removed it so I could
> trouble shoot this problem, but I didn't re-add it and the system
booted
> fine, Xen is working.  Is there something that depends on this?
>
> 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.

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

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).

 -George

Jerry,
Refer to console=hvc0 from (https://wiki.xen.org/wiki/Xen_FAQ_Console)which
is dedicated for domO after Xen kernel loaded, so if it hangs it means
somewhere the booting process with xen is not right.

Suggest you to close look at the dmsg or log for debugging. Hope that helps
and cheers.

Xlord

On Wed, May 17, 2017 at 11:26 AM, Jerry <jerryubi at gmail.com> wrote:
> Howdy,
>
> I recently went through a frustrating experience trying to get Xen 4
> running on a CentOS 7 system.  After a fresh install, fully updating the
> system, rebooting, then trying to install Xen4CentOS it would fail to boot
> into the 4.9 kernel, sitting there with a blinking cursor indefinitely.
>
> I thought it was a failure with grub, but it turns out this was set in the
> grub config:
>
> GRUB_CMDLINE_LINUX_XEN_REPLACE_DEFAULT="console=hvc0 ..."
>
> So the kernel was loading successfully, but it was failing boot properly
> (hanging because it couldn't find the / file system, but that's a
different
> story). Boot messages were not being displayed on my console because of the
> console=hvc0 setting, hampering the troubleshooting process.
>
> What is the purpose of setting console=hvc0?  I removed it so I could
> trouble shoot this problem, but I didn't re-add it and the system
booted
> fine, Xen is working.  Is there something that depends on this?
>
> 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.
>
> Thanks,
> Jerry
>
> _______________________________________________
> CentOS-virt mailing list
> CentOS-virt at centos.org
> https://lists.centos.org/mailman/listinfo/centos-virt
>
>
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On Tue, May 16, 2017 at 10:26 PM, Jerry <jerryubi at gmail.com> wrote:
> Howdy,
>
> I recently went through a frustrating experience trying to get Xen 4
> running on a CentOS 7 system.  After a fresh install, fully updating the
> system, rebooting, then trying to install Xen4CentOS it would fail to boot
> into the 4.9 kernel, sitting there with a blinking cursor indefinitely.
>
...

Check the BIOS to see if Hyperthreading is disabled. If so, enable it. I
had some Dell server (Dell R710s and R610s) that I finally figured out
would not boot like you describe when HT is off for some reason.
PJ
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On Wed, May 17, 2017 at 3:43 AM, PJ Welsh <pjwelsh at gmail.com> wrote:
> On Tue, May 16, 2017 at 10:26 PM, Jerry <jerryubi at gmail.com>
wrote:
>
>> Howdy,
>>
>> I recently went through a frustrating experience trying to get Xen 4
>> running on a CentOS 7 system.  After a fresh install, fully updating
the
>> system, rebooting, then trying to install Xen4CentOS it would fail to
boot
>> into the 4.9 kernel, sitting there with a blinking cursor indefinitely.
>>
> ...
>
> Check the BIOS to see if Hyperthreading is disabled. If so, enable it. I
> had some Dell server (Dell R710s and R610s) that I finally figured out
> would not boot like you describe when HT is off for some reason.
> PJ
>
I found what my issue was with it not booting, when the 4.9 kernel was
installed it didn't include the mpt3sas driver.  I posted the solution to
the mailing list earlier with the subject:

4.9 kernel fails to boot because it didn't have the mpt3sas module

This post is just asking what the purpose of console=hvc0 is.

Thanks,
Jerry
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On Wed, May 17, 2017 at 3:02 AM, Mark L Sung <xlord.sl at gmail.com>
wrote:
> Jerry,
> Refer to console=hvc0 from (https://wiki.xen.org/wiki/
> Xen_FAQ_Console)which is dedicated for domO after Xen kernel loaded, so
> if it hangs it means somewhere the booting process with xen is not right.
>
I might be missing something, but that article makes no mention of
using hvc0 on dom0.

Jerry
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On Wed, May 17, 2017 at 2:39 AM, George Dunlap <dunlapg at umich.edu>
wrote:
> On Wed, May 17, 2017 at 4:26 AM, Jerry <jerryubi at gmail.com> 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.

Thanks,
Jerry
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