linux-kernel - Re: [PATCH] x86-64: espfix for 64-bit mode *PROTOTYPE*

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Message-ID: <535F5173.5030301@zytor.com>
Date:	Tue, 29 Apr 2014 00:14:59 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	"H. Peter Anvin" <hpa@...ux.intel.com>,
	Andrew Lutomirski <amluto@...il.com>
CC:	comex <comexk@...il.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Alexander van Heukelum <heukelum@...tmail.fm>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	Borislav Petkov <bp@...en8.de>,
	Arjan van de Ven <arjan.van.de.ven@...el.com>,
	Brian Gerst <brgerst@...il.com>,
	Alexandre Julliard <julliard@...ehq.com>,
	Andi Kleen <andi@...stfloor.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH] x86-64: espfix for 64-bit mode *PROTOTYPE*

On 04/28/2014 09:36 PM, H. Peter Anvin wrote:
> 
> There are still things that need fixing: we need to go through the
> espfix path even when returning from NMI/MC (which fortunately can't
> nest with taking an NMI/MC on the espfix path itself, since in that case
> we will have been interrupted while running in the kernel with a kernel
> stack.)
> 
> (Cc: Rostedt because of the NMI issue.)
> 

NMI is fine: we go through irq_return except for nested NMI.  There are
only three IRETs in the kernel (irq_return, nested_nmi_out, and the
early trap handler) and all of them are good.

I think we just need to clean up the PV aspects of this and then we
should be in good shape.  I have done a bunch of cleanups to the
development git tree.

I'm considering making 16-bit segment support a EXPERT config option for
both 32- and 64-bit kernels, as it seems like a bit of a waste for
embedded systems which don't need this kind of backward compatibility.
Maybe that is something that can be left for someone else to implement
if they feel like it.

	-hpa

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