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    SubjectRe: [PATCH 6/7] [RFC] enable early TLBs for BG/P
    On Thu, May 19, 2011 at 7:39 PM, Benjamin Herrenschmidt
    <> wrote:
    > On Wed, 2011-05-18 at 16:24 -0500, Eric Van Hensbergen wrote:
    >> BG/P maps firmware with an early TLB
    > That's a bit gross. How often do you call that firmware in practice ?
    > Aren't you better off instead inserting a TLB entry for it when you call
    > it instead ? A simple tlbsx. + tlbwe sequence would do. That would free
    > up a TLB entry for normal use.

    Well, it depends on who you talk to. The production software BG/P
    guys use the firmware
    constantly, its the primary interface to the networks, the console,
    and the management software
    which runs the machine. As such the IO Node guys, the Compute Node
    Kernel guys and the
    ZeptoOS guys use it quite a bit. The kittyhawk guys on the other hand
    barely use it at all, in fact
    I believe they do all the interaction with it during uboot and then shut it off.

    IIRC, the sticky question is RAS support, there are certain things it
    wants to jump to firmware
    to deal with and expects things to be mapped an pinned into memory.
    Furthermore, I think it
    may make assumptions about where in the TLB the mappings are. Since
    the kittyhawk guys
    obviously ignore this by shutting it down, its not clear just how
    important this is. I'm game to
    try the dynamic mapping as you suggest if you would prefer it.

    Its worth mentioning that I believe with BG/Q, the plan is to rely on
    the firmware even more
    extensively, but I haven't looked at any of the code yet to verify
    whether or not this is true.


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