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




> On May 31, 2019, at 12:20 PM, Jann Horn <jannh@xxxxxxxxxx> wrote:
> 
> 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?)

Thanks. I thought that your experience with TLB page-freeing bugs may
be valuable, and you frequently find my mistakes. ;-)

Yes, speculatively executed cached reads from the I/O memory are a concern.
IIRC they caused #MC on AMD. If page-tables are not changes, but only PTEs
are changed, I don’t see how it can be a problem. I also looked at the MTRR
setting code, but I don’t see a concrete problem.