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Message-ID: <20110407080825.GF24879@elte.hu>
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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