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    SubjectRe: /dev/mem "unbounded?"
    On 11/09/2011 02:38 PM, Andrei Warkentin wrote:
    > Hi Peter,
    >
    > ----- Original Message -----
    >>
    >> That would be incorrect behavior, though, except perhaps for the
    >> range
    >> that cannot be addressed by the processor. It is explicitly
    >> permitted
    >> to address ranges that does not have addresses mapped to it.
    >
    > There is a current mechanism for restricting access to a subset
    > of addresses, and it is used to enforce < 1MB accesses on x86
    > if CONFIG_STRICT_DEVMEM. This could be extended.
    >

    Well, that mechanism is broken. The way it *should* work is that any
    region which is system RAM should be denied access, and the rest is
    accessible. The current behavior is a hack due to the behavior of some
    old versions of Xorg, but that has long been fixed.

    > Do you think there is any value in specifying something like
    > CONFIG_DEV_MEM_ONLY_CLAIMED, which would only allow accesses
    > until the end of the last range claimed with request_region (but
    > could, of course, well be unmapped). This will allow accesses to
    > any unclaimed "holes" in between. I.e., if you have 0-100m claimed,
    > followed by 300-700m claimed, then reading /dev/mem will work up
    > until you reach 700m.

    No. If you don't know what you're accessing, you should not be touching
    /dev/mem under any circumstances. Odds are that even if you're only
    reading, there are registers with side effects in there somewhere.

    -hpa





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