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 09:24:53 -0800
From:   Arjan van de Ven <arjan@...ux.intel.com>
To:     Peter Zijlstra <peterz@...radead.org>
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

> The idea is simple, do what we do for virt. Don't send IPI's to CPUs
> that don't need them (in virt's case because the vCPU isn't running, in
> our case because we're not in fact running a user process), but mark the
> CPU as having needed a TLB flush.

I am really uncomfortable with that idea.
You really can't run code safely on a cpu where the TLBs in the CPU are invalid
or where a CPU that does (partial) page walks would install invalid PTEs either
through actual or through speculative execution.

(in the virt case there's a cheat, since the code is not actually running
there isn't a cpu with TLBs live. You can't do that same cheat for this case)