Re: [Qemu-devel] Loading ELF binaries with very high base addresses

[Prashant Vaibhav <qemu@mercurysquad.com>]

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Thanks Marc and Alex,

Yes it was 0x4<<60, missed a zero. I didn't know about the virtual regions,
I assumed it was flat 64-bit.

For now I am going with Alex's suggestion and forcing the mmap to happen at
a lower memory address, starting at mmap_min_addr. The guest_base is then
large, causing a wraparound.

This way also the region bits won't be disregarded, but will make 8
contiguous regions giving a flat 64-bit space. I don't know which types of
programs make use of different regions simultaneously, will probably have to
think about it later, but for now this is good enough.

~Prashant



On Tue, Jul 12, 2011 at 10:44 PM, "Marc Lörner" <loerner@gmx.de> wrote:

> Hello Prashant,
> first of all your "0x4<<64" is wrong it's "0x4<<60".
> In Volume 2 of the IASDM page 2:46 you see that these three upper bits
> correspond to the 8 virtual regions (here: region 2).
> So maybe you can just disregard these bits and use the rest as new offset
> to an faked guest_base that fits your needs (e.g. somewhere in your
> process space)?
>
> Regards,
> Marc
>
> >Hello,
> >
> >I am working on target-ia64, but am stuck during ia64 ELF loading.
> >
> >Referring to function "probe_guest_base()" in linux-user/elfload.c around
> >line 1350, called from around line 1484 --
> >
> >When the main binary is being mmap'd, the host address and guest address
> >should ideally be the same. If they're not, a linear search is done by
> >increasing the host_address by one page and trying the mmap again. The
> >(positive) offset is then saved.
>
> >The problem occurs with ia64 binaries, which typically start at
> >0x4000000000000000  (i.e 0x4<<64). At least on my x86_64 host machine,
> >mmap'ing at this address fails. The real_address is of the order of
> >0x8<<32. Needless to say, increasing host_address and trying again will
> >never reach a lower address to map at. Further, I cannot make it relocate
> >to a lower host address because the offset (guest_base) is an unsigned >int
> and so the relocation can only happen by a positive offset.
> >
> >Because of this it is not possible to load any ELF binaries which start
> >at such high memory addresses. I can tailor an elf binary to start at a
> >lower base address, which might work for that specific case, but I >suspect
> most existing ia64 binaries start at 0x4<<64 by convention. Also, >the
> "hiaddr" is read from elf header which again is set to 0x4<<64 + some
> >value.
> >
> >The existing code works fine with x86_64, for example, because the
> >binaries are typically starting at 0x40000, which is easily mmap'd at
> >first try.
> >
> >Any ideas on a workaround?
> >
> >~Prashant
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