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xen-devel
RE: [Xen-devel] Memory limits?
The 4GB limitation mentioned here is for guests. What about the host?
Does the same limitation apply to the host?
Yes. Which is to say, memory over and above 4Gig won't be usable (although
the system would obviously still boot).
If yes... Could you tell me
how much effort is involved in enabling PAE mode in 32-bit Xen?
I think the concensus was that it wasn't hugely difficult (relative to
other tasks in the VM system, i.e. would probably still require an
experienced kernel hacker) but that it would be a bit too time consuming to
justify the effort required.
What
components would need to be modified?
Take a look under xen/arch/x86/x86/mm.c. You'd need to update this to deal
with the i686 PAE case, as well as the existing i386 and x86_64 page table
layouts. I heard there was some commonality between i686 PAE and x86_64
page tables but I don't know how much.
You'd also need to poke around in the Linux memory management code. You'll
have to pull in PAE support from x86 Linux, modifying the files to Xen-ify
them.
You might need to modify some of the userspace tools to understand PAE and
probably some other bits and bobs within Xen.
It's worth emphasising again that Xen *will* support memory > 4gig on the
upcoming x86_64 port (as well as the IA64 port and other suitable ports).
Cheers,
Mark
Aravindh
-----Original Message-----
From: xen-devel-admin@xxxxxxxxxxxxxxxxxxxxx
[mailto:xen-devel-admin@xxxxxxxxxxxxxxxxxxxxx] On Behalf Of Mark
Williamson
Sent: Monday, February 21, 2005 5:57 PM
To: xen-devel@xxxxxxxxxxxxxxxxxxxxx
Cc: Rene Auberger
Subject: Re: [Xen-devel] Memory limits?
> I am relatively new to Xen and have a question regarding memory: I was
not
> able to find out what the limit of memory is I can assign to a guest
> system. How much memory can I assign to each guest:
> * not more than physical memory in the machine?
Yes, Xen doesn't do swapping. If swapping is needed, the guests should
be
configured to do it themselves.
> * not more than physical memory minus memory used by dom0 and other
doms?
Correct. Bear in mind Xen uses some memory too.
> * not more than 4GB?
Correct, on x86 we don't support the PAE36 mode required to support more
than
4GB. This is not an insurmountable technical limitation, it's just that
it's
not deemed worth the development time. On other systems (e.g. x86_64),
larger memories does not have the same problems.
Cheers,
Mark
> * something totally different?
>
> By the way: I am using SuSE 9.2 with the Xen packages from
> http://www.suse.de/~garloff/linux/xen/ - don't know if that has any
> influence on which answer applies to my question.
>
> Rene
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