Re: [Xen-ia64-devel] Virtual mem map

[Tristan Gingold <Tristan.Gingold@xxxxxxxx>]

Le Vendredi 06 Janvier 2006 22:40, Magenheimer, Dan (HP Labs Fort Collins) a 
écrit :
> > I am currently thinking about virtual mem map.
> > In Linux, the virtual mem map is (surprise) virtually mapped.
> > In Xen, we can use the same approach or we can manually cut
> > the mem map into
> > several pieces using a mechanism similar to paging.
[...]
> I think you will need to explain a little bit more what you
> mean by "a mechanism similar to paging" before it will
> be possible to comment.  Paging, to me, means there is some
> kind of swap drive or backing store to allow more "virtual"
> pages than "physical" pages.
[I have just describe it in my previous mail].

> I spent a lot of time recently digging through the physical
> memory management code of Linux/ia64.  It is very messy because
> it has to support a wide variety of physical memory layouts.
> And it makes surprising choices that throw away huge chunks of
> physical memory (anything that doesn't fit conveniently in
> a "granule").
Yes, this is very surprising.  At least 16MB of memory are lost!  Maybe Xen 
can use this memory for itself ?

>  Getting this all working on multiple machines
> will probably be a big challenge.  It might be best to use
> Linux code that is known to work on many machines.
Sure.
On the other side, Xen (almost) don't use ptc/itc for itself.  

> I agree with your concern though that taking TLB misses when
> looking up a page struct in Xen is likely to cause performance
> problems and some difficult bugs.  It might be worthwhile to
> put some counters in to see how frequently the memmap is
> accessed and code some defensive bounds checks to ensure
> wild accesses are immediately flagged with a BUG rather than
> resulting in random memory accesses.
I agree.
Tristan.


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