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Date: Tue, 29 Apr 2014 00:14:59 -0700
From: "H. Peter Anvin" <hpa@zytor.com>
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To: "H. Peter Anvin" <hpa@linux.intel.com>,
        Andrew Lutomirski <amluto@gmail.com>
CC: comex <comexk@gmail.com>,
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        Linus Torvalds <torvalds@linux-foundation.org>,
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        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
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        Borislav Petkov <bp@alien8.de>,
        Arjan van de Ven <arjan.van.de.ven@intel.com>,
        Brian Gerst <brgerst@gmail.com>,
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        Andi Kleen <andi@firstfloor.org>, Thomas Gleixner <tglx@linutronix.de>,
        Steven Rostedt <rostedt@goodmis.org>
Subject: Re: [PATCH] x86-64: espfix for 64-bit mode *PROTOTYPE*
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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