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    SubjectRe: Dealing with the NMI mess
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    On 07/09/2015 10:19, Maciej W. Rozycki wrote:
    >> > Essentially the ICE breakpoint instruction enters SMM mode?
    > I didn't do stuff at the probe firmware level so I can't say for sure,
    > but my gut feeling is the debug mode is indeed very close if not the same
    > as SMM. I think duplicating the logic would be an unnecessary waste of
    > silicon.

    I researched SMM a bit recently in order to implement it in KVM, and the
    best source of folklore seems to be http://www.rcollins.org/ddj (which I
    also have on paper :)).

    The author there says that SMM design was roughly based on the 386's
    probe/ICE mode design, but it's actually separate. Most notably, on the
    386 the state save areas almost mirror each other, but when I say
    mirror... I do mean mirror: directions are reversed, and what is on top
    for probe mode is on bottom for SMM. :)

    In addition, AMD tried reusing ICE mode for SMM, and was sued by Intel
    who actually won the lawsuit. I couldn't find more information about
    the lawsuit.

    It's probably diverged more and more over time, for example because SMM
    is now considered security-sensitive while probe mode isn't. In
    addition, the same DDJ article says that Pentium JTAG probe mode
    "doesn't resemble SMM at all, doesn't use a state save map, or even
    execute any code of its own", whatever that means.

    Paolo

    > And obviously it's any cause of #DB that enters this mode. The probe can
    > also request it right at the exit from the reset state, so that you can
    > debug software (e.g BIOS startup) right from the reset vector. You don't
    > need working RAM for that.


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