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 19:18:52 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Arjan van de Ven <arjan@...ux.intel.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        LKML <linux-kernel@...r.kernel.org>,
        KarimAllah Ahmed <karahmed@...zon.de>,
        Andi Kleen <ak@...ux.intel.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        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>,
        Radim Krcmar <rkrcmar@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Tom Lendacky <thomas.lendacky@....com>, X86 ML <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 09:04:21AM -0800, Andy Lutomirski wrote:
> I haven't tried to fully decipher the patch, but I think the idea is
> wrong.  (I think it's the same wrong idea that Rik and I both had and
> that I got into Linus' tree for a while...)  The problem is that it's
> not actually correct to run indefinitely in kernel mode using stale
> cached page table data.  The stale PTEs themselves are fine, but the
> stale intermediate translations can cause the CPU to speculatively
> load complete garbage into the TLB, and that's bad (and causes MCEs on
> AMD CPUs).

Urggh.. indeed :/

> I think we only really have two choices: tlb_defer_switch_to_init_mm()
> == true and tlb_defer_switch_to_init_mm() == false.  The current
> heuristic is to not defer if we have PCID, because loading CR3 is
> reasonably fast.

I just _really_ _really_ hate idle drivers doing leave_mm(). I don't
suppose limiting the !IPI case to just the idle case would be correct
either, because between waking from idle and testing our 'should I have
invalidated' bit it can (however unlikely) speculate into stale TLB
entries too..