Date: Sat, 2 Apr 2016 22:47:52 +0200
From: Borislav Petkov <bp@alien8.de>
To: Andy Lutomirski <luto@amacapital.net>
Cc: Andy Lutomirski <luto@kernel.org>, X86 ML <x86@kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>, KVM list <kvm@vger.kernel.org>,
        Arjan van de Ven <arjan@linux.intel.com>,
        xen-devel <Xen-devel@lists.xen.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH v5 3/9] x86/head: Move early exception panic code into
 early_fixup_exception
Message-ID: <20160402204752.GC2538@pd.tnic>
References: <cover.1459605520.git.luto@kernel.org>
 <4085070316fc3ab29538d3fcfe282648d1d4ee2e.1459605520.git.luto@kernel.org>
 <20160402183919.GA2538@pd.tnic>
 <CALCETrUJVDpzNQMsY=_jWNOMer+TdW2p4wES4OBCfYU7gq3fbg@mail.gmail.com>
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On Sat, Apr 02, 2016 at 01:13:37PM -0700, Andy Lutomirski wrote:
> Given that I this isn't really a regression with my patches (it
> probably never worked much better on 32-bit and the regs never would
> have shown at all on 64-bit),

You're right. That thing calls printk *and* early_printk, WTF:

#ifdef CONFIG_EARLY_PRINTK

	call early_printk
	...

	call dump_stack

	...

	call __print_symbol

those last two call printk. Great.

> I propose a different approach: make
> printk work earlier.  Something like:
> 
> if (early) {
>     early_printk(args);
> }
> 
> or early_vprintk or whatever.
> 
> If the cost of a branch mattered, this could be alternative-patched
> out later on, but that seems silly.  I also bet that a more sensible
> fallback could be created in which printk would try to use an early
> console if there's no real console.

So how about this:

printk() does

	vprintk_func = this_cpu_read(printk_func);

and that's

DEFINE_PER_CPU(printk_func_t, printk_func) = vprintk_default

I guess we can make that function be early_printk-something and once
printk is initialized, we overwrite it with vprintk_default.

Elegant and no need for if branches and alternatives.

Hmmm.

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.