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Re: [Qemu-devel] [PATCH] PPC: Fix linker scripts on ppc hosts

From: Paul Brook
Subject: Re: [Qemu-devel] [PATCH] PPC: Fix linker scripts on ppc hosts
Date: Wed, 14 Dec 2011 02:24:17 +0000
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> >> IIRC mmap'ing files would break with 32-on-64, but I'd have to check up
> >> on the details. I ended up passing MAP_32BIT to all linux-user mmap
> >> calla for 32-on-x86_64, but that doesn't work with -R.
> > 
> > Hmm, I thought we'd fixed that.  It's the reason h2g_valid exists.
> > 
> > Either way it should definitely work with -R.  I specifically added that
> > to avoid problems with the host mmap picking inconvenient addresse.
> > 
> > MAP_32BIT is an unconsionable hack, and doesn't exist on other 64-bit
> > hosts.
> Yes, I agree. I just sat down once more and checked if I could get it
> working with -R and everything seems fine so far.
> Is there any particular reason we're not reserving all of the possible 32
> bit address space for 32-on-64 guests? The way it's today with the default
> being completely broken and -R fixing everything if you give it a large
> enough value is ... suboptimal.

-R doesn't actually fix everything :-)

In particular if you use ulimit -v then there's a good chance it will break 
completely.  Having said that, it's not hard to check ulimit -v first.

I still think the current config should work.  The automagical guest_base 
isn't quite as reliable as -R (the guest and host spaces probably overlap) but 
it should be comparable to 32-on-32.

If we do default to -R 4G for 32-on-64, then there's a good argument for 
enabling it by default for all combinations.  For 32-bit hosts you can either 
try to guess the size of the host user address space, or just start by trying 
3G and if that doesn't work progressively fall back to 2G, 1G, then current 

For 64-on-64 maybe pick a large value, remebering that the reservation 
probably isn't completely free.


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