Messages in this thread | | | Date | Tue, 07 Oct 2008 08:28:08 -0700 | From | Jeremy Fitzhardinge <> | Subject | Re: [patch 3/7] x86, cpa: make the kernel physical mapping initialization a two pass sequence |
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Suresh Siddha wrote: > Jeremy, hi. This dependency is not documented or explicitly called anywhere > in the mm/init_64.c code. I would have expected to see a big comment near this > kind of code :( >
Indeed yes. I've explained it in various places, including commit comments, but there should be a comment right there in the code.
> It is not just the NX bit that we change. For DEBUG_PAGEALLOC, we want > use 4k pages instead of large page mappings during the identity mapping > (as this will clean some of the cpa pool code avoiding the cpa and hence > the page allocations for splitting the big pages from interrupt context's). > In this case will will split the static large page mappings. >
Well, that's OK. We just need to preserve the original page permissions when fragmenting the large mappings. (This isn't a case that affects Xen, because it will already be 4k mappings.)
>> 3. The actual implementation is pretty ugly; adding a global variable >> and hopping about with goto does not improve this code. >> > > This is very early init code and I can't be fancy like calling cpa() > which need mm to be up and running.
Well, is there any urgency to set NX that early? It might catch some early bugs, but there's no urgent need.
> And also, cpa's on individual chunks > for entire identity mapping will make the boot slow. >
Really? Why? How slow?
>> it cause real failures? Could we revert this patch and address the >> problem some other way? Which app note is this, BTW? The one I have on >> hand, "TLBs, Paging-Structure Caches, and Their Invalidation", Apr 2007, >> does not seem to mention this restriction. >> > > http://developer.intel.com/design/processor/applnots/317080.pdf > Section 6 page 26 >
Ah, OK. I have the first version of this document which does not mention this. It would be good to explicitly cite this document by name in the comments.
>> Xen with this code in place (touching this code is always non-trivial). >> I haven't looked into it in depth yet, but there's a few stand out "bad >> for Xen" pieces of code here. (And I haven't tested 32-bit yet.) >> >> Quick rules for keeping Xen happy here: >> >> 1. Xen provides its own initial pagetable; the head_64.S one is >> unused when booting under Xen. >> 2. Xen requires that any pagetable page must always be mapped RO, so >> we're careful to not replace an existing mapping with a new one, >> in case the existing mapping is a pagetable one. >> 3. Xen never uses large pages, and the hypervisor will fail any >> attempt to do so. >> > > Thanks for this info. Will get back to you tomorrow. >
Great. Also, do you think you'll have a chance to look at unifying the 32 and 64 bit code (where 32 uses the 64-bit version)?
Thanks, J
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