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

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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> 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.
So the hvc0 is to use the PV console driver to pipe all the messages to the Xen
one.

And Xen is configured to use the serial console (com1=115200,8n1).

Which means that all you Linux bootup info should be piped to that.

But Linux is pretty quiet unless you add 'loglevel=10' or
'debug' on the
Linux command line.
> 
> Thanks,
> Jerry
> _______________________________________________
> CentOS-virt mailing list
> CentOS-virt at centos.org
> https://lists.centos.org/mailman/listinfo/centos-virt

On Wed, May 17, 2017 at 8:25 AM, Konrad Rzeszutek Wilk <
konrad.wilk at oracle.com> wrote:
> > 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.
>
> So the hvc0 is to use the PV console driver to pipe all the messages to
> the Xen one.
>
> And Xen is configured to use the serial console (com1=115200,8n1).
>
> Which means that all you Linux bootup info should be piped to that.
>
>
So how would I properly configure it to still write to tty without
disabling hvc0?  Perhaps something like this?

GRUB_CMDLINE_LINUX_XEN_REPLACE_DEFAULT="console=hvc0,tty earlyprintk=xen
nomodeset"

Looks like I have some learning to do.  Do you happen to know of a good
article explaining how console redirection works?

> But Linux is pretty quiet unless you add 'loglevel=10' or
'debug' on the
> Linux command line.
>
>
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On Wed, May 17, 2017 at 4:20 PM, Jerry <jerryubi at gmail.com>
wrote:
> 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.
See below -- it sends the output to Xen; Xen will then forward it to
the serial, the screen, or both.
> 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.
It's the CentOS Xen package setting this.  But the Xen option
"console=com1,tty" should make it such that Xen sends its output
*both* to the serial line, *and* the monitor.

I take it you're not seeing any Xen output at all on your IPMI console?

 -George

On Wed, May 17, 2017 at 8:44 AM, George Dunlap <dunlapg at umich.edu>
wrote:
> On Wed, May 17, 2017 at 4:20 PM, Jerry <jerryubi at gmail.com> wrote:
> > 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.
>
> It's the CentOS Xen package setting this.  But the Xen option
> "console=com1,tty" should make it such that Xen sends its output
> *both* to the serial line, *and* the monitor.
>
> I take it you're not seeing any Xen output at all on your IPMI console?
>
>  -George
>
The boot messages are suppressed, but if it boots successfully the login
prompt shows up. If I remove console=hvc0 both the boot messages and login
prompt show up.

Note: the IPMI console I'm looking at is just the local monitor being
redirected.  It isn't a special device/driver.

I'll try what Konrad is suggesting, setting this:

console=hvc0 console=tty

And will see what happens.  It'll be a few hours until I can try it out.

Thank you to every one that has responded, I appreciate the help.

Jerry.
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