linux-kernel - Re: [PATCH v5 3/9] x86/head: Move early exception panic code into early_fixup

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Date:	Mon, 4 Apr 2016 21:38:03 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	Peter Zijlstra <peterz@...radead.org>,
	Andy Lutomirski <luto@...capital.net>
Cc:	Jan Kara <jack@...e.cz>, Paolo Bonzini <pbonzini@...hat.com>,
	xen-devel <Xen-devel@...ts.xen.org>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	X86 ML <x86@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Petr Mladek <pmladek@...e.cz>, KVM list <kvm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v5 3/9] x86/head: Move early exception panic code into
 early_fixup_exception

On Mon, Apr 04, 2016 at 06:00:42PM +0200, Peter Zijlstra wrote:
> On Mon, Apr 04, 2016 at 08:32:21AM -0700, Andy Lutomirski wrote:
> 
> > Adding locking would be easy enough, wouldn't it?
> 
> See patch in this thread..
> 
> > But do any platforms really boot a second CPU before switching to real
> > printk? 
> 
> I _only_ use early_printk() as printk() is a quagmire of fail :-)

And since I'm the king of minimalistic changes... this below
works. However, the problem is that we need to pull up all that
early_param parsing in order to enable the early console, i.e.
"earlyprintk=ttyS0,115200" cmdline parsing.

And we can do all that after having setup the IDT. So I'd need to do
some early dancing with cmdline_find_option_bool(boot_command_line, ...
in asm or so. Need to think about it more.

---
diff --git a/arch/x86/kernel/head64.c b/arch/x86/kernel/head64.c
index 1f4422d5c8d0..ad534226653b 100644
--- a/arch/x86/kernel/head64.c
+++ b/arch/x86/kernel/head64.c
@@ -130,6 +130,13 @@ static void __init copy_bootdata(char *real_mode_data)
 	}
 }
 
+static int _early_printk(const char *fmt, va_list args)
+{
+	early_printk(fmt, args);
+
+	return 0;
+}
+
 asmlinkage __visible void __init x86_64_start_kernel(char * real_mode_data)
 {
 	int i;
@@ -164,6 +171,10 @@ asmlinkage __visible void __init x86_64_start_kernel(char * real_mode_data)
 	load_idt((const struct desc_ptr *)&idt_descr);
 
 	copy_bootdata(__va(real_mode_data));
+	parse_early_param();
+	this_cpu_write(printk_func, _early_printk);
+
+	printk("This is a test!\n");
 
 	/*
 	 * Load microcode early on BSP.
diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
index 2367ae07eb76..998d6c675549 100644
--- a/arch/x86/kernel/setup.c
+++ b/arch/x86/kernel/setup.c
@@ -882,6 +882,7 @@ void __init setup_arch(char **cmdline_p)
 	 */
 	__flush_tlb_all();
 #else
+	this_cpu_write(printk_func, vprintk_default);
 	printk(KERN_INFO "Command line: %s\n", boot_command_line);
 #endif
 
diff --git a/include/linux/printk.h b/include/linux/printk.h
index 9ccbdf2c1453..97df81c97b2f 100644
--- a/include/linux/printk.h
+++ b/include/linux/printk.h
@@ -169,6 +169,7 @@ void __init setup_log_buf(int early);
 __printf(1, 2) void dump_stack_set_arch_desc(const char *fmt, ...);
 void dump_stack_print_info(const char *log_lvl);
 void show_regs_print_info(const char *log_lvl);
+int vprintk_default(const char *fmt, va_list args);
 #else
 static inline __printf(1, 0)
 int vprintk(const char *s, va_list args)

-- 
Regards/Gruss,
    Boris.

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