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SubjectRe: [patch 3/7] x86, cpa: make the kernel physical mapping initialization a two pass sequence
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.
> 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)?


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