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Subject: Re: [patch RFC 5/5] x86/speculation: Add basic speculation control
 code
To: Andrea Arcangeli <aarcange@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>
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Cc: David Woodhouse <dwmw2@infradead.org>,
        Jiri Kosina <jikos@kernel.org>,
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        Borislav Petkov <bp@alien8.de>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Andi Kleen <ak@linux.intel.com>,
        Greg KH <gregkh@linuxfoundation.org>,
        Andy Lutomirski <luto@kernel.org>
From: Dave Hansen <dave.hansen@intel.com>
Message-ID: <1610a587-fe9e-28bd-dcd1-b9ec940c07ef@intel.com>
Date: Wed, 10 Jan 2018 06:59:54 -0800
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On 01/10/2018 06:10 AM, Andrea Arcangeli wrote:
> Tim and Dave please comment too, Tim you originally wrote that code
> that leaves IBRS always on and never toggles it in the kernel entry
> point so you must know full well if Arjan is correct that you must
> toggle IBRS every time you enter kernel and in turn ibrs_enabled 2
> isn't valid mode.

Hi Andrea,

The "writing IBRS=1 acts as a barrier when it is already IBRS=1"
behavior is something which I misunderstood in the past.  Thanks, Arjan,
for clearing it up.