console: introduce console=none option

console: introduce console=none option

[Andrew Cooper]

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XenServer by default boots without a serial console (buggy hardware
reasons) and dom0 displays a splash screen.  Unfortunately, having Xen
writing to the vga text area looks ugly whilst dom0 is trying to set up
non-text mode and display  the splash screen.

We have been using "console=" to prevent this behavior for a while, but
presented herewith is a patch to fix the problem correctly.

~Andrew


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Console: introduce console=none command line parameter

Currenty, not specifying 'console=<foo>' on the command line causes
Xen to default to 'vga'.  Alternativly, the user can explicitly
specifiy 'console=vga|com1|com2'.

However, there is no way to specify that neither vga nor serial should
be used.  Specifying 'console=' does have the effect that neither vga
nor serial is set up, but at the cost of an "Bad console= option ''"
warning.

Therefore, expliticly support a 'console=none' option which does not
set up vga and does not set up serial, but does not trigger the bad
console warning.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>

diff -r a2a8089b1ffb xen/drivers/char/console.c
--- a/xen/drivers/char/console.c
+++ b/xen/drivers/char/console.c
@@ -549,6 +549,8 @@ void __init console_init_preirq(void)
             p++;
         if ( !strncmp(p, "vga", 3) )
             vga_init();
+        else if ( !strncmp(p, "none", 4) )
+            continue;
         else if ( strncmp(p, "com", 3) ||
                   (sercon_handle = serial_parse_handle(p)) == -1 )
         {

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Re: console: introduce console=none option

[Jan Beulich]

>>> On 26.01.12 at 13:19, Andrew Cooper <andrew.cooper3@citrix.com> wrote:
> XenServer by default boots without a serial console (buggy hardware
> reasons) and dom0 displays a splash screen.  Unfortunately, having Xen
> writing to the vga text area looks ugly whilst dom0 is trying to set up
> non-text mode and display  the splash screen.
> 
> We have been using "console=" to prevent this behavior for a while, but
> presented herewith is a patch to fix the problem correctly.

While I don't mind the patch, I'm completely confused by the description:
Where is it that Xen writes to VGA text area after control was passed to
Dom0?

Jan

Re: console: introduce console=none option

[Andrew Cooper]

On 26/01/12 14:41, Jan Beulich wrote:
>>>> On 26.01.12 at 13:19, Andrew Cooper <andrew.cooper3@citrix.com> wrote:
>> XenServer by default boots without a serial console (buggy hardware
>> reasons) and dom0 displays a splash screen.  Unfortunately, having Xen
>> writing to the vga text area looks ugly whilst dom0 is trying to set up
>> non-text mode and display  the splash screen.
>>
>> We have been using "console=" to prevent this behavior for a while, but
>> presented herewith is a patch to fix the problem correctly.
> While I don't mind the patch, I'm completely confused by the description:
> Where is it that Xen writes to VGA text area after control was passed to
> Dom0?
>
> Jan

I have not debugged it that much as I was looking for a clean solution
to our current hack of "console=" (and I have some rather more serious
deadlock bugs to debug), but it all Xen printk's are going into the VGA
text area, even after dom0 has started.  It is possible that Xen is
still writing into the text area after dom0 has switched VGA mode, but I
have no proof of this one way or the other.

-- 
Andrew Cooper - Dom0 Kernel Engineer, Citrix XenServer
T: +44 (0)1223 225 900, http://www.citrix.com

Re: console: introduce console=none option

[Jan Beulich]

>>> On 26.01.12 at 15:58, Andrew Cooper <andrew.cooper3@citrix.com> wrote:
> On 26/01/12 14:41, Jan Beulich wrote:
>>>>> On 26.01.12 at 13:19, Andrew Cooper <andrew.cooper3@citrix.com> wrote:
>>> XenServer by default boots without a serial console (buggy hardware
>>> reasons) and dom0 displays a splash screen.  Unfortunately, having Xen
>>> writing to the vga text area looks ugly whilst dom0 is trying to set up
>>> non-text mode and display  the splash screen.
>>>
>>> We have been using "console=" to prevent this behavior for a while, but
>>> presented herewith is a patch to fix the problem correctly.
>> While I don't mind the patch, I'm completely confused by the description:
>> Where is it that Xen writes to VGA text area after control was passed to
>> Dom0?
>>
>> Jan
> 
> I have not debugged it that much as I was looking for a clean solution
> to our current hack of "console=" (and I have some rather more serious
> deadlock bugs to debug), but it all Xen printk's are going into the VGA
> text area, even after dom0 has started.  It is possible that Xen is
> still writing into the text area after dom0 has switched VGA mode, but I
> have no proof of this one way or the other.

Just take a look at vga_endboot() - the output routine gets pointed to
vga_noop_puts() unless vgacon_keep. Beyond that point nothing
can possibly get printed to the VGA text screen, or if it does, then I'd
suspect there's some other change in XenServer that makes it so.

Jan

Re: console: introduce console=none option

[Tim Deegan]

At 16:00 +0000 on 26 Jan (1327593636), Jan Beulich wrote:
> >>> On 26.01.12 at 15:58, Andrew Cooper <andrew.cooper3@citrix.com> wrote:
> > On 26/01/12 14:41, Jan Beulich wrote:
> >>>>> On 26.01.12 at 13:19, Andrew Cooper <andrew.cooper3@citrix.com> wrote:
> >>> XenServer by default boots without a serial console (buggy hardware
> >>> reasons) and dom0 displays a splash screen.  Unfortunately, having Xen
> >>> writing to the vga text area looks ugly whilst dom0 is trying to set up
> >>> non-text mode and display  the splash screen.
> >>>
> >>> We have been using "console=" to prevent this behavior for a while, but
> >>> presented herewith is a patch to fix the problem correctly.
> >> While I don't mind the patch, I'm completely confused by the description:
> >> Where is it that Xen writes to VGA text area after control was passed to
> >> Dom0?
> >>
> >> Jan
> > 
> > I have not debugged it that much as I was looking for a clean solution
> > to our current hack of "console=" (and I have some rather more serious
> > deadlock bugs to debug), but it all Xen printk's are going into the VGA
> > text area, even after dom0 has started.  It is possible that Xen is
> > still writing into the text area after dom0 has switched VGA mode, but I
> > have no proof of this one way or the other.
> 
> Just take a look at vga_endboot() - the output routine gets pointed to
> vga_noop_puts() unless vgacon_keep. Beyond that point nothing
> can possibly get printed to the VGA text screen, or if it does, then I'd
> suspect there's some other change in XenServer that makes it so.

IIRC in some XenServer versions the bootloader also puts up a
splashscreen; that may be causing some confusion.

Tim.

Re: console: introduce console=none option

[Ian Campbell]

On Thu, 2012-01-26 at 16:00 +0000, Jan Beulich wrote:
> >>> On 26.01.12 at 15:58, Andrew Cooper <andrew.cooper3@citrix.com> wrote:
> > On 26/01/12 14:41, Jan Beulich wrote:
> >>>>> On 26.01.12 at 13:19, Andrew Cooper <andrew.cooper3@citrix.com> wrote:
> >>> XenServer by default boots without a serial console (buggy hardware
> >>> reasons) and dom0 displays a splash screen.  Unfortunately, having Xen
> >>> writing to the vga text area looks ugly whilst dom0 is trying to set up
> >>> non-text mode and display  the splash screen.
> >>>
> >>> We have been using "console=" to prevent this behavior for a while, but
> >>> presented herewith is a patch to fix the problem correctly.
> >> While I don't mind the patch, I'm completely confused by the description:
> >> Where is it that Xen writes to VGA text area after control was passed to
> >> Dom0?
> >>
> >> Jan
> > 
> > I have not debugged it that much as I was looking for a clean solution
> > to our current hack of "console=" (and I have some rather more serious
> > deadlock bugs to debug), but it all Xen printk's are going into the VGA
> > text area, even after dom0 has started.  It is possible that Xen is
> > still writing into the text area after dom0 has switched VGA mode, but I
> > have no proof of this one way or the other.
> 
> Just take a look at vga_endboot() - the output routine gets pointed to
> vga_noop_puts() unless vgacon_keep. Beyond that point nothing
> can possibly get printed to the VGA text screen, or if it does, then I'd
> suspect there's some other change in XenServer that makes it so.

IIRC the original purpose of this patch was to stop Xen from displaying
anything at all on the VGA, not just after dom0 handover but right from
start of day.

This was so that the splash screen (placed by the bootloader) didn't get
overwritten until dom0 comes up and takes over with the boot progress
meter thing.

Maybe that's changed in the meantime but that's my recollection.

Ian.

Re: console: introduce console=none option

[Andrew Cooper]

On 26/01/12 16:00, Jan Beulich wrote:
>>>> On 26.01.12 at 15:58, Andrew Cooper <andrew.cooper3@citrix.com> wrote:
>> On 26/01/12 14:41, Jan Beulich wrote:
>>>>>> On 26.01.12 at 13:19, Andrew Cooper <andrew.cooper3@citrix.com> wrote:
>>>> XenServer by default boots without a serial console (buggy hardware
>>>> reasons) and dom0 displays a splash screen.  Unfortunately, having Xen
>>>> writing to the vga text area looks ugly whilst dom0 is trying to set up
>>>> non-text mode and display  the splash screen.
>>>>
>>>> We have been using "console=" to prevent this behavior for a while, but
>>>> presented herewith is a patch to fix the problem correctly.
>>> While I don't mind the patch, I'm completely confused by the description:
>>> Where is it that Xen writes to VGA text area after control was passed to
>>> Dom0?
>>>
>>> Jan
>> I have not debugged it that much as I was looking for a clean solution
>> to our current hack of "console=" (and I have some rather more serious
>> deadlock bugs to debug), but it all Xen printk's are going into the VGA
>> text area, even after dom0 has started.  It is possible that Xen is
>> still writing into the text area after dom0 has switched VGA mode, but I
>> have no proof of this one way or the other.
> Just take a look at vga_endboot() - the output routine gets pointed to
> vga_noop_puts() unless vgacon_keep. Beyond that point nothing
> can possibly get printed to the VGA text screen, or if it does, then I'd
> suspect there's some other change in XenServer that makes it so.
>
> Jan

I have just been debugging this, and found that it got up to my debug
message of switching back to vga_noop_puts().

I know that this was called before dom0 started booting, but that it
appeared as if it was being written while dom0 was booting

It turns out that the problem is that after vesa_endboot zeros the
linear framebuffer, it does not call lfb_flush(), meaning that an sfence
does not occur, and presumably the actual linear framebuffer still has
Xen console text in it when dom0 takes over and sets it up.

Hacking an sfence into vesa_endboot() fixes the issue completely.

I shall roll a patch using lfb_flush() and post it shortly.

-- 
Andrew Cooper - Dom0 Kernel Engineer, Citrix XenServer
T: +44 (0)1223 225 900, http://www.citrix.com