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Subject: Re: x86_64: tss_struct layout does not match comments  !?
From: Richard Kennedy <richard@rsk.demon.co.uk>
To: Ingo Molnar <mingo@elte.hu>
Cc: lkml <linux-kernel@vger.kernel.org>,
       the arch/x86 maintainers <x86@kernel.org>
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	 <20080703113925.GA5594@elte.hu>
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Date: Thu, 03 Jul 2008 12:57:01 +0100
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On Thu, 2008-07-03 at 13:39 +0200, Ingo Molnar wrote:
> * Richard Kennedy <richard@rsk.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

No problem.
Richard