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    SubjectRe: [RFC] prevent "dd if=/dev/mem" crash
    David Mosberger <> wrote:
    > >>>>> On Fri, 17 Oct 2003 17:49:55 -0700, Andrew Morton <> 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?

    Possibly not. I thought that simply oopsing the kernel was a bit rude, and
    fixing ia32 to not do that was relatively simple.

    We should, within reason, handle it as gracefully as possible, yes?

    > 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).

    If the not-present memory is marked cacheable and/or writeback then yes,
    but that would be an odd thing to do, wouldn't it?

    It the memory is mapped noncacheable then a synchronous error on a read
    sounds reasonable. A synchronous error on a write would assume that the
    noncacheability affects the write buffers and IIRC that usually doesn't

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