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

[Mark Williamson <mark.williamson@xxxxxxxxxxxx>]

> > That said, in principle it ought to be alright.  32-on-64 even fully
> > supports glibc's TLS, which even 32-on-32 doesn't (this is because of
> > slightly arcane implementation details in x86.  Really.  The mind
> > boggles.).
>
> I won't ask as I don't need my brain exploded on Friday afternoon ;-)

Well, it's really very simple.  Basically what happens is *bang* *bang* 
*splat*[1]

OK, maybe you were right :-)

> > If you're running a 64-bit Xen, I think you also have the potential to
> > run 64-bit paravirt guests, and 64-bit HVM guests in addition to all the
> > stuff you could run on 32-bit Xen.
>
> Yes that is certainly of future interest.
>
> > At some point it should be possible to run a 32-bit dom0, but I'm not
> > sure if that's allowed yet.
>
> Well that is how I tested it so I guess it is possible ;-)

Well, more particularly, I don't know if a 32-bit dom0 will boot 64-bit 
guests, which may or may not matter for you.  I think it's on the roadmap.

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).

Cheers,
Mark

[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 :-)

-- 
Dave: Just a question. What use is a unicyle with no seat?  And no pedals!
Mark: To answer a question with a question: What use is a skateboard?
Dave: Skateboards have wheels.
Mark: My wheel has a wheel!

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