linux-kernel - Re: [RFT/PATCH v2 1/6] x86-64: Clean up vdso/kernel shared variables

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Date:	Thu, 7 Apr 2011 10:08:25 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Andy Lutomirski <luto@....EDU>
Cc:	x86@...nel.org, Thomas Gleixner <tglx@...utronix.de>,
	Andi Kleen <andi@...stfloor.org>, linux-kernel@...r.kernel.org
Subject: Re: [RFT/PATCH v2 1/6] x86-64: Clean up vdso/kernel shared variables


* Andy Lutomirski <luto@....EDU> wrote:

> Variables that are shared between the vdso and the kernel are
> currently a bit of a mess.  They are each defined with their own
> magic, they are accessed differently in the kernel, the vsyscall page,
> and the vdso, and one of them (vsyscall_clock) doesn't even really
> exist.
> 
> This changes them all to use a common mechanism.  All of them are
> delcared in vvar.h with a fixed address (validated by the linker
> script).  In the kernel (as before), they look like ordinary
> read-write variables.  In the vsyscall page and the vdso, they are
> accessed through a new macro VVAR, which gives read-only access.
> 
> The vdso is now loaded verbatim into memory without any fixups.  As a
> side bonus, access from the vdso is faster because a level of
> indirection is removed.

Ok, that's a pretty nice consolidation and speedup.

This layout:

> +DECLARE_VVAR(0, volatile unsigned long, jiffies)
> +DECLARE_VVAR(128, struct vsyscall_gtod_data, vsyscall_gtod_data)
> +DECLARE_VVAR(256, int, vgetcpu_mode)

Is spread out too much, using up several separate cachelines with nothing else 
on them. Why not pack it tightly, with natural alignment taken into 
consideration?

Thanks,

	Ingo
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