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Re: [Qemu-devel] Updated >2G memory patch

Subject: Re: [Qemu-devel] Updated >2G memory patch
From: "J. Mayer"
Date: Sun, 30 Sep 2007 01:16:57 +0200
On Sat, 2007-09-29 at 23:43 +0100, Paul Brook wrote:
> > > Also note that changing variables from int to long have strictly no
> > > impact on 32 bits host machines, then won't help emulating more than 2
> > > GB of RAM. Another variable type (target_phys_addr_t ?) should be used
> > > instead.
> >
> > This patch should be restricted to 64-bit hosts. I don't think it's
> > useful to emulate a 64-bit target with huge amounts of virtual and
> > physical address space on a 32-bit host.

My feeling is that if it's restricted to 64 bits host, then it's a patch
for geeks only, that brings no useful feature to the main end-users. In
the real world, most people are still running in 32 bits mode.

> IMHO Huge amounts of virtual address space can definitely be useful, even if 
> you don't have ram to back it.
> Huge amounts of physical address space is less immediately useful, though in 
> practice you have to emulate whatever real hardware provides. If you're 
> emulating a machine with a 40+ bit physical address space, there's a fair 
> chance your guest OS will decide to scatter a relatively small set of 
> resources over the whole address space.

I don't agree too much with your opinion, because what I can see is that
PowerPC 64 machines (at least IBM ones) tend to use the 62 bits physical
address space provided by the architecture. If I remember well, there is
at least one PPC64 architecture where the highest bits are used to split
the physical address space between memory, memory-mapped IO,
devices, ...
I'm quite sure there are other 64 bits architecture that have the same
requirement of a huge physical address space, then beeing able to handle
it in Qemu seems to be very useful, much more than trying to emulate a
huge amount of RAM, and is needed in a very near future.

> I agree there's no point trying to emulate >2G ram on a 32-bit host, but 
> physical address space and ram are two very different things.
> For example I have a cpu that has a "bitbanded" memory region. This expands 
> each bit of real ram to a whole 32-bit word, effectively turning a word 
> load/store into an atomic bit operation. Currently it's only used for 
> relatively small address ranges, but it's a good example of a situation where 
> the physical address space is much larger than ram.

I don't see why it would be useless to emulate huge amount of RAM on 32
bits hosts. If you try to register more than a few gigabytes of memory,
there are great chances that the host machine won't have the physical
RAM to handle it at once, so a page swap mechanism will have to be
implemented. Then, I see no difference in using it on a 32 bits hosts or
a 64 bits ones.


J. Mayer <[email protected]>
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