qemu-system-alpha (was: Re: [Qemu-devel] [PATCH] Fix extlh instruction on Alpha)

[Brian Wheeler]

On Sun, 2009-09-20 at 21:20 -0500, Rob Landley wrote:
> On Thursday 17 September 2009 14:28:52 Vince Weaver wrote:
> > The extlh instruction on Alpha currently doesn't work properly.
> > It's a combination of a cut/paste bug (16 where it should be 32) as well
> > as a "shift by 64" bug.
> >
> > This improves on an earlier patch that used labels, conditional jumps,
> > and local variables.  Thanks go especially to Aurelien Jarno and Andreas
> > Schwab who have a much better eye for bit-wise TCG optimization than I do.
> >
> > Vince
> 
> Any idea how hard it would be to whip up a qemu-system-alpha emulation?  I 
> note that several real-world alpha boards were essentially just a PC with a 
> different processor.  

Yep.  The ES40 used the ALI1543 chipset and was pretty pc-ish is most
respects.  There were a couple of DEC specific chips on board (such as
an IOMMU).  

But the hardware isn't the real problem, I don't think.  I think the
real problems (for QEMU, at least) are the PAL and SRM.

The PAL adds several os-specific pseudo-instructions to the instruction
set.  MILO has a linux palcode, but no other open source ones are
available that I know of.  Without VMS or Tru64 Palcode you're limited
to linux.  I think the BSDs use the Tru64 Palcode.  I suppose its
possible to have the palcode routines hard-coded into qemu, though you'd
need several sets of them (one for each supported OS type)

There doesn't seem to be an open source SRM which can be used to boot
the machine and provide the initial user interface.  MILO also performs
this function, but its hard to build and pretty old.  

I believe that charon axp solves this problem by having their own
mini-implementation of the srm (and presumably the PAL) which only
supports setting/viewing the environment and booting.  The real SRM has
lots of features that aren't really needed on emulated hardware.

> I'd like to boot Alpha Linux on qemu, and it doesn't seem like there's _that_ 
> much more to do.  But last I asked (a couple years ago) I was told the Alpha 
> protected mode stuff wasn't implemented yet...

There were patches last year for an es40, but they never made it to the
main tree...and weren't quite finished.