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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@xxxxxxxxxx> wrote:
> [ +Jann Horn ]
> > On May 31, 2019, at 3:57 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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?)