linux-kernel - Re: [RFC PATCH v2 11/12] x86/mm/tlb: Use async and inline messages for flushing

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Date:   Fri, 31 May 2019 21:20:21 +0200
From:   Jann Horn <jannh@...gle.com>
To:     Nadav Amit <namit@...are.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...el.com>,
        Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [RFC PATCH v2 11/12] x86/mm/tlb: Use async and inline messages
 for flushing

On Fri, May 31, 2019 at 8:29 PM Nadav Amit <namit@...are.com> wrote:
>
> [ +Jann Horn ]
>
> > On May 31, 2019, at 3:57 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> >
> > On Thu, May 30, 2019 at 11:36:44PM -0700, Nadav Amit wrote:
> >> When we flush userspace mappings, we can defer the TLB flushes, as long
> >> the following conditions are met:
> >>
> >> 1. No tables are freed, since otherwise speculative page walks might
> >>   cause machine-checks.
> >>
> >> 2. No one would access userspace before flush takes place. Specifically,
> >>   NMI handlers and kprobes would avoid accessing userspace.
[...]
> A #MC might be caused. I tried to avoid it by not allowing freeing of
> page-tables in such way. Did I miss something else? Some interaction with
> MTRR changes? I’ll think about it some more, but I don’t see how.

I don't really know much about this topic, but here's a random comment
since you cc'ed me: If the physical memory range was freed and
reallocated, could you end up with speculatively executed cached
memory reads from I/O memory? (And if so, would that be bad?)