Re: [Xen-users] Xen 3.1 - 32 vs 64 bit hypervisor

[Tim Post <tim.post@xxxxxxxxxxx>]

On Fri, 2007-06-08 at 17:38 +0100, Mark Williamson wrote:

> 32-bit PV guests potentially are actually faster on 64-bit than 64-bit PV 
> guests.  Due to <more arcane x86isms> I believe the system call interface can 
> be faster for a 32-bit PV guest than a 64-bit guest.  So conceivably it's 
> more efficient to run a 32-bit dom0 (maybe it won't *actually* matter, I 
> don't know).

I'm now thoroughly confused. You mean, if you could run a 64 bit guest /
32 bit hypervisor, right? 

> [1] Actually, it's very simple as long as you don't expect it to sound 
> remotely reasonable.  It *is* x86 after all :-)  Xen wants to be in the top 
> part of the address space.  32-bit glibc wants to mess with the top of the 
> address space in order to implement TLS.  On 32-bit hosts this conflicts.  On 
> a 64-bit host, Xen stashes itself at the top of the virtual address space; 
> because it's a 64-bit host this is waaaaaaaaaaaay higher than a 32-bit guest 
> is capable of addressing.  Therefore, it's fine to let glibc play with the 
> top of the 32-bit space - it just can't get at Xen :-)

That's why I'm wondering what you meant above, or does this completely
change if you run a 32 bit hypervisor with a 64 bit guest?

So if a 64 bit tree fell in 32 bit address space ... , oh sod it never
mind. Just tell us when it works and we'll say "Thank you."

Best,
--Tim


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