linux-kernel - Re: [GIT pull] x86 mpx support for 3.19

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Date:	Wed, 10 Dec 2014 20:14:44 -0600
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>, x86@...nel.org,
	Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [GIT pull] x86 mpx support for 3.19

Thomas Gleixner <tglx@...utronix.de> writes:

> Linus,
>
> please pull the latest x86-mpx-for-linus git tree from:
>
>    git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git x86-mpx-for-linus
>
> This enables support for x86 MPX:
>
>   MPX is a new debug feature for bound checking in user space. It
>   requires kernel support to handle the bound tables and decode the
>   bound violating instruction in the trap handler.

I some really dumb questions.

Given that mpx is both architecture and cpu specific why use prctl?
I would think arch_prctl would be a much more appropriate place to
expose this logic.

Why don't you have a call to let an application query to see if mpx
management is enabled?  I am trying to imagine how checkpoint/restart is
going to be supported for mpx applications.  If I can't even query if
mpx is enabled I don't have a clue how we could save this state and
duplicate it in another process on another machine.

> +Adding new prctl commands
> +-------------------------
> +
> +Two new prctl commands are added to enable and disable MPX bounds tables
> +management in kernel.
> +
> +155	#define PR_MPX_ENABLE_MANAGEMENT	43
> +156	#define PR_MPX_DISABLE_MANAGEMENT	44
> +
> +Runtime library in userspace is responsible for allocation of bounds
> +directory. So kernel have to use XSAVE instruction to get the base
> +of bounds directory from BNDCFG register.
> +
> +But XSAVE is expected to be very expensive. In order to do performance
> +optimization, we have to get the base of bounds directory and save it
> +into struct mm_struct to be used in future during PR_MPX_ENABLE_MANAGEMENT
> +command execution.
> +


Eric
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