Re: [Xen-devel] Sparc32/64 porting effort, interest?

[Tristan Gingold <Tristan.Gingold@xxxxxxxx>]

Le Mercredi 09 Août 2006 14:31, Jimi Xenidis a écrit :
> On Aug 9, 2006, at 2:10 AM, Tristan Gingold wrote:
> > Le Samedi 05 Août 2006 23:05, Blue Swirl a écrit :
[...]
> > Big questions!
> >
> > Which hardware are you targetting ?  Sparc or SparcV9?
> >
> > Sparc doesn't have a three level protection mechanism.  So adding a
> > VMM can be
> > tricky.
>
> Actually its easier, unlike those "three level" guys SPARC (and PPC)
> has a clean separation for what can and cannot be done in the 2 modes
> supported.  So I'd be pushing the Linux kernel into user space, using
> really simple hvcalls() to virtualize memory then write a nice
> Illeagal/protected instruction recovery mechanism.
> After that you can start extending the paravirt ops to take care of
> stuff.
Just a note from xen/ia64 history:
ia64 has 4 levels (almost like x86).  The first implementation of Xen was done 
without recompiling the Linux kernel: it was simply patched (some 
level-sensitive instruction were replaced by traps).  This allows to focus on 
Xen only.  This maybe simpler.

> *** NOTE: Some PPC chips do provide a third hypervisor mode, but
> thats simply for some extra performance ***
>
> Linux should not be too hard because the Linux Sparc code has
> excellent abstractions and with the new Niagra stuff, you may find
> the abstractions are "just right".
>
> > PowerPc people may have ideas to handle this case.
>
> We have an experimental mode that pushes the kernel into user mode,
> it requires Xen to craft initial memory spaces like a real-mode
> mapping and kernel-virtual mapping that is usually V-maps-P.
> One thing that we have played with is the ability to create "virtual
> supervisor register" space in the last virtual page (0-4k), that has
> increased performance.
> You should be able to do the same with SPARC since you can have linux
> "fixup" all 'RD <reg>, Rx' instructions to  'LD r3,-<reg * 8>'.
> ** and you guys thought the simm13 mode was useless :)
On xen/ia64 we also use memory-mapped registers.  They are often faster than 
genuine registers!

[...]
> > At the last XenSummit, there was at least one people from Sun.  I
> > don't know
> > wether other developers are interested.
>
> No Sparc people, I was looking. :(
You didn't look hard enough ;-)  He was interesting in Xen for Niagara.

Tristan.

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