linux-kernel - Re: [PATCH RFC] x86_64: per-cpu memory for user-space

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Message-ID: <20140914140639.GO5387@tassilo.jf.intel.com>
Date:	Sun, 14 Sep 2014 07:06:39 -0700
From:	Andi Kleen <ak@...ux.intel.com>
To:	Konstantin Khlebnikov <koct9i@...il.com>
Cc:	x86@...nel.org, linux-kernel@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Dmitry Vyukov <dvyukov@...gle.com>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH RFC] x86_64: per-cpu memory for user-space

On Sat, Sep 13, 2014 at 06:35:34PM +0400, Konstantin Khlebnikov wrote:
> This patch implements user-space per-cpu memory in the same manner as in
> kernel-space: each cpu has its own %gs base address. On x86_64 %fs is used
> for thread local storage, %gs usually is free.
> 
> User-space application cannot prevent preemption but x86 read-modify-write
> operations are atomic against interrupts and context switches. Thus percpu
> counters, ring-buffer cursors, per-cpu locks and other cool things might
> be implemented in a very efficient way.

Do you have some concrete examples for the more complex operations?

It seems to me the limitation to a simple instruction will be very limiting
for anything more complicated than a counter. Also it's not even
clear how someone would implement retry (short of something like kuchannel)

Of course it wouldn't be a problem with TSX transactions, but it's not
clear they need it.

The other problem with the approach is, how would cpu hotplug
be handled?

> By the way, newer Intel cpus have even faster instructions for
> changing %fs/%gs, but they are still not supported by the kernel.

Patch kits are pending.

-Andi
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