linux-kernel - Re: [RFC PATCH 1/2] x86/ibpb: Skip IBPB when we switch back to same user process

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Date:   Thu, 25 Jan 2018 14:50:55 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Arjan van de Ven <arjan@...ux.intel.com>
Cc:     Tim Chen <tim.c.chen@...ux.intel.com>,
        linux-kernel@...r.kernel.org,
        KarimAllah Ahmed <karahmed@...zon.de>,
        Andi Kleen <ak@...ux.intel.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        Ashok Raj <ashok.raj@...el.com>,
        Asit Mallick <asit.k.mallick@...el.com>,
        Borislav Petkov <bp@...e.de>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        David Woodhouse <dwmw@...zon.co.uk>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "H . Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        Janakarajan Natarajan <Janakarajan.Natarajan@....com>,
        Joerg Roedel <joro@...tes.org>,
        Jun Nakajima <jun.nakajima@...el.com>,
        Laura Abbott <labbott@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>, rkrcmar@...hat.com,
        Thomas Gleixner <tglx@...utronix.de>,
        Tom Lendacky <thomas.lendacky@....com>, x86@...nel.org
Subject: Re: [RFC PATCH 1/2] x86/ibpb: Skip IBPB when we switch back to same
 user process

On Thu, Jan 25, 2018 at 05:21:30AM -0800, Arjan van de Ven wrote:
> > 
> > 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.

The intel_idle code does, not the idle code. This is squirreled away in
some driver :/

> (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)

Hurmph. I'd rather fix that some other way than leave_mm(), this is
piling special on special.