linux-kernel - Re: [PATCH][RESEND] x86: Do not write to VGA memory space if CONFIG_VGA

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Date:	Tue, 06 Apr 2010 13:17:27 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	guenter.roeck@...csson.com
CC:	linux-kernel@...r.kernel.org, penberg@...helsinki.fi,
	mingo@...hat.com, x86@...nel.org
Subject: Re: [PATCH][RESEND] x86: Do not write to VGA memory space if CONFIG_VGA_CONSOLE
 is undefined

On 04/05/2010 03:12 PM, Guenter Roeck wrote:
>>
>> I'm unhappy about it, because *those early messages shouldn't exist in
>> the first place*.  It seems to be an indication that we're invoking
>> setup_early_printk() too late.  The whole playing around with max_xpos
>> and max_ypos instead of using boot_params.screen_info directly is
>> particularly bleacherous.
>>
>> I would at least like to see if the improper invocation of
>> early_printk() can be avoided.
>>
> There are several such invocations.
> 
> 1) arch/x86/kernel/head_64.S:
> ENTRY(early_idt_handler)
> ...
> 	leaq early_idt_msg(%rip),%rdi
> 	call early_printk
> 
> This displays "PANIC: early exception %02lx rip %lx:%lx error %lx cr2 %
> lx\n" and subsequently calls dump_stack. The handler is initialized from
> x86_64_start_kernel().
> 
> 2) arch/x86/kernel/head64.c:x86_64_start_kernel():
> 	if (console_loglevel == 10)
> 		early_printk("Kernel alive\n");
> 
> 3) init/main.c: start_kernel()
> 	printk(KERN_NOTICE "%s", linux_banner);
>     and
> 	printk(KERN_NOTICE "Kernel command line: %s\n", boot_command_line);
> 
> 4) arch/x86/kernel/setup.c:setup_arch()
> 	Several.
> 
> After that I gave up looking.
> 
> Not sure if or how those can be avoided.
> 
> Moving setup_early_printk() into x86_64_start_kernel() might be an
> option, but that would require much more significant changes.
> 

Okay... that leaves a few very ugly options.

I think spewing onto a potentially uninitialized (or not even present)
serial port is worse than losing the messages.  It seems to be pretty
much a no-brainer to have:

	if (boot_params.screen_info.orig_video_isVGA != 1)
		return;

... in early_vga_write(), and is something we should do regardless.

Calling early_serial_write() before early_serial_init() is distinctly
not safe... depending on boot conditions you might end up with a glacial
boot.

Therefore, the only sensible way to get the early messages out is really
to push setup_early_printk() as early as possible.

	-hpa


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