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Subject: Re: [RFC PATCH 1/2] x86/ibpb: Skip IBPB when we switch back to same
 user process
To:     Peter Zijlstra <peterz@infradead.org>,
        Tim Chen <tim.c.chen@linux.intel.com>
Cc:     linux-kernel@vger.kernel.org,
        KarimAllah Ahmed <karahmed@amazon.de>,
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        Andrea Arcangeli <aarcange@redhat.com>,
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        Ashok Raj <ashok.raj@intel.com>,
        Asit Mallick <asit.k.mallick@intel.com>,
        Borislav Petkov <bp@suse.de>,
        Dan Williams <dan.j.williams@intel.com>,
        Dave Hansen <dave.hansen@intel.com>,
        David Woodhouse <dwmw@amazon.co.uk>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "H . Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@redhat.com>,
        Janakarajan Natarajan <Janakarajan.Natarajan@amd.com>,
        Joerg Roedel <joro@8bytes.org>,
        Jun Nakajima <jun.nakajima@intel.com>,
        Laura Abbott <labbott@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>, rkrcmar@redhat.com,
        Thomas Gleixner <tglx@linutronix.de>,
        Tom Lendacky <thomas.lendacky@amd.com>, x86@kernel.org
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From:   Arjan van de Ven <arjan@linux.intel.com>
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Date:   Thu, 25 Jan 2018 05:21:30 -0800
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> 
> This means that 'A -> idle -> A' should never pass through switch_mm to
> begin with.
> 
> Please clarify how you think it does.
> 

the idle code does leave_mm() to avoid having to IPI CPUs in deep sleep states
for a tlb flush.

(trust me, that you really want, sequentially IPI's a pile of cores in a deep sleep
state to just flush a tlb that's empty, the performance of that is horrific)