linux-kernel - Re: [PATCH 16/17] x86-64: Remove the PDA

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Date:	Sun, 18 Jan 2009 17:13:32 +0900
From:	Tejun Heo <tj@...nel.org>
To:	Brian Gerst <brgerst@...il.com>
CC:	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 16/17] x86-64: Remove the PDA

Hello, Brian.

Brian Gerst wrote:
>> How about something like the following?
>>
>> #define CANARY_OFFSET   40
>> #define CANARY_SIZE     8
>>
>> DECLARE_PER_CPU(unsigned long, stack_canary);
>>
>> and in linker script,
>>
>> PERCPU_VADDR_PREALLOC(0, :percpu, CANARY_OFFSET + CANARY_SIZE)
>> per_cpu__stack_canary = __per_cpu_start + CANARY_OFFSET;
>>
> 
> The thing I don't like about the prealloc method is that it puts the
> page-aligned variables at the end.  This leaves a gap which is
> unavailable for dynamic allocations.  Stealing 48 bytes from the
> bottom of the irqstack (which is 16k) keeps the page-aligned section
> at the start.  It's really no different than how the thread_info
> structure sits at the bottom of the process stack.
> 
> How about something like:
> union irq_stack_union {
> 	char irq_stack[IRQSTACKSIZE];
> 	struct {
> 		char pad[40];
> 		unsigned long stack_canary;
> 	}
> };
> 
> That documents the overlay better, and avoids having to touch the
> linker script.

I have no objection as long as it's sufficiently documented.

Thanks.

-- 
tejun
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