Re: [PATCH] x86: Stop relying on magic jmp behavior for early_idt_handlers

From: H. Peter Anvin
Date: Thu May 21 2015 - 20:50:36 EST


On 05/21/2015 05:38 PM, Andy Lutomirski wrote:
> On Thu, May 21, 2015 at 5:32 PM, H. Peter Anvin <hpa@xxxxxxxxx> wrote:
>> On 05/21/2015 05:17 PM, Andy Lutomirski wrote:
>>>
>>> diff --git a/arch/x86/include/asm/segment.h b/arch/x86/include/asm/segment.h
>>> index 5a9856eb12ba..4bbe0eb24d7e 100644
>>> --- a/arch/x86/include/asm/segment.h
>>> +++ b/arch/x86/include/asm/segment.h
>>> @@ -231,12 +231,21 @@
>>> #define TLS_SIZE (GDT_ENTRY_TLS_ENTRIES* 8)
>>>
>>> #ifdef __KERNEL__
>>> -#ifndef __ASSEMBLY__
>>>
>>> -extern const char early_idt_handlers[NUM_EXCEPTION_VECTORS][2+2+5];
>>> +/*
>>> + * early_idt_handlers is an array of entry points. For simplicity, it's
>>> + * a real array. We allocate nine bytes for each entry: two one-byte
>>> + * push instructions and a five-byte jump in the worst case.
>>> + */
>>> +#define EARLY_IDT_HANDLER_STRIDE 9
>>> +#ifndef __ASSEMBLY__
>>> +extern const char early_idt_handlers[NUM_EXCEPTION_VECTORS][EARLY_IDT_HANDLER_STRIDE];
>>> #ifdef CONFIG_TRACING
>>> # define trace_early_idt_handlers early_idt_handlers
>>> #endif
>>> +#endif
>>> +
>>> +#ifndef __ASSEMBLY__
>>>
>>
>> Please get rid of the unnecessary #ifndef/#endif pair.
>
> I did it to preserve logical nesting. I didn't want the pile of
> early_idt_handlers declarations and comments to have the side affect
> of starting an #ifdef block.
>
> If you still think I should change it, I'll change it.
>

Please change it. We generally don't do that kind of logical nesting,
it is just clutter.

-hpa

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