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    SubjectRe: [RFC] prevent "dd if=/dev/mem" crash
    >>>>> On Fri, 17 Oct 2003 17:49:55 -0700, Andrew Morton <akpm@osdl.org> said:

    Andrew> We _want_ to be able to read mmio ranges via /dev/mem, don't
    Andrew> we? I guess it has never come up because everyone uses
    Andrew> kmem.

    I just don't see how making a "dd if=/dev/mem" safe and allowing
    access to arbitrary physical memory can go to together. Given that
    /dev/mem _is_ being used for accessing mmio space, is it really worth
    bothering trying to make such a "dd" safe?

    Andrew> If the hardware doesn't give the system programmer a choice
    Andrew> then the hardware is poorly designed, surely?

    Emh, we're talking about _physical_ memory accesses here. AFAIK,
    failures on physical memory accesses are never signaled with
    synchronous faults (not on any reasonably modern high performance
    architecture, at least). Loads probably _could_ be signalled
    synchronously, but consider stores: would you really want to wait with
    retiring a store until it has made it all the way to some slow ISA
    device? I think not (IN/OUT do that). No, modern CPUs check the
    TLB/page-table and if that check passes, they'll _assume_ the memory
    access will complete without errors. If it doesn't, they signal an
    asynchronous failure (e.g., via an MCA).

    --david
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