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Subject: Re: [PATCH 30/35] x86/speculation: Use Indirect Branch Prediction
 Barrier in context switch
To: Peter Zijlstra <peterz@infradead.org>,
 David Woodhouse <dwmw2@infradead.org>, Thomas Gleixner <tglx@linutronix.de>,
 Josh Poimboeuf <jpoimboe@redhat.com>
Cc: linux-kernel@vger.kernel.org, Dave Hansen <dave.hansen@intel.com>,
 Ashok Raj <ashok.raj@intel.com>, Andy Lutomirski <luto@kernel.org>,
 Linus Torvalds <torvalds@linux-foundation.org>,
 Greg KH <gregkh@linuxfoundation.org>, Andrea Arcangeli
 <aarcange@redhat.com>, Andi Kleen <ak@linux.intel.com>,
 Arjan Van De Ven <arjan.van.de.ven@intel.com>,
 Dan Williams <dan.j.williams@intel.com>, Paolo Bonzini
 <pbonzini@redhat.com>, Jun Nakajima <jun.nakajima@intel.com>,
 Asit Mallick <asit.k.mallick@intel.com>, Jason Baron <jbaron@akamai.com>
References: <20180118134800.711245485@infradead.org>
 <20180118140153.257709600@infradead.org>
From: Tim Chen <tim.c.chen@linux.intel.com>
Message-ID: <3a3c86d1-d15a-ec98-a9c0-e7a43f5c9cc5@linux.intel.com>
Date: Thu, 18 Jan 2018 16:38:32 -0800
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On 01/18/2018 05:48 AM, Peter Zijlstra wrote:
>
>+		/*
>+		 * Avoid user/user BTB poisoning by flushing the branch predictor
>+		 * when switching between processes. This stops one process from
>+		 * doing spectre-v2 attacks on another process's data.
>+		 */
>+		indirect_branch_prediction_barrier();
>+

Some optimizations can be done here to avoid overhead in barrier call.

For example, don't do the barrier if prev and next mm are
the same.  If the two process trust each other, or the new process
already have rights to look into the previous process,
the barrier could be skipped.

Tim