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Date: Thu, 19 May 2011 20:21:54 -0500 From: Eric Van Hensbergen <ericvh@...il.com> To: Benjamin Herrenschmidt <benh@...nel.crashing.org> Cc: linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org, bg-linux@...ts.anl-external.org Subject: Re: [PATCH 6/7] [RFC] enable early TLBs for BG/P On Thu, May 19, 2011 at 7:39 PM, Benjamin Herrenschmidt <benh@...nel.crashing.org> 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. -eric -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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