linux-kernel - Re: x86_64: tss_struct layout does not match comments !?

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Message-ID: <20080703113925.GA5594@elte.hu>
Date:	Thu, 3 Jul 2008 13:39:25 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Richard Kennedy <richard@....demon.co.uk>
Cc:	lkml <linux-kernel@...r.kernel.org>,
	the arch/x86 maintainers <x86@...nel.org>
Subject: Re: x86_64: tss_struct layout does not match comments  !?


* Richard Kennedy <richard@....demon.co.uk> wrote:

> Hi Ingo,
> 
> the comments in the definition of tss_struct suggests is should be
> cacheline aligned ( or 256 byte aligned ? ) :- 
> 
> |struct tss_struct {
> |....
> |	/*
> |	 * Pad the TSS to be cacheline-aligned (size is 0x100):
> |	 */
> |	unsigned long		__cacheline_filler[35];
> |	/*
> |	 * .. and then another 0x100 bytes for the emergency kernel stack:
> |	 */
> |	unsigned long		stack[64];
> |
> |} __attribute__((packed));
> 
> However on a 64 bit build the size of tss_struct is 9136,
> cacheline_filler is 280 and stack size is 512 at offset 8624.
> None of which are cacheline aligned.
> 
> I'm guessing this isn't what was intended.
> 
> do you know what the original intention was ?
> 
> 1. struct tss_struct{...} __cacheline_aligned;
> or just
> 2. struct tss_struct {
> ...
> long stack[64] __cacheline_aligned;
> }

#1 is the intent - because each CPU has a separate TSS. init_tss.stack 
is really just an emergency static stack we have in place for very early 
exceptions.

i think the __cacheline_filler could be removed safely. Mind sending a 
patch for that?

	Ingo
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