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Date:   Thu, 25 Jan 2018 19:18:52 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Andy Lutomirski <luto@kernel.org>
Cc:     Arjan van de Ven <arjan@linux.intel.com>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        LKML <linux-kernel@vger.kernel.org>,
        KarimAllah Ahmed <karahmed@amazon.de>,
        Andi Kleen <ak@linux.intel.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        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>,
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        Thomas Gleixner <tglx@linutronix.de>,
        Tom Lendacky <thomas.lendacky@amd.com>, X86 ML <x86@kernel.org>
Subject: Re: [RFC PATCH 1/2] x86/ibpb: Skip IBPB when we switch back to same
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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..