Re: [Xen-devel] [PATCH v5 3/9] x86/head: Move early exception panic code into early_fixup_exception

[Andy Lutomirski <luto@xxxxxxxxxxxxxx>]

[cc Jan Kara]

On Sat, Apr 2, 2016 at 1:47 PM, Borislav Petkov <bp@xxxxxxxxx> wrote:
> 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.

Jan, IIRC you were looking at printk recently-ish.  Any thoughts here?

--Andy

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