Re: [Qemu-devel] Removal of some target CPU macros

[Jocelyn Mayer]

On Wed, 2007-11-07 at 19:32 +0100, Fabrice Bellard wrote:
> I noticed that some target CPUs macros have been added while they do not
> seem necessary. I don't like that because it introduces more #ifdefs
> which prevent making a version supporting simultaneously all the CPUs.
> 
> In particular I saw the following:
> 
> - TARGET_MIPSN32 : it is always combined with TARGET_MIPS64 in
> target-mips/. If its only usage is to select a different Linux ABI, then
> I suggest keeping TARGET_MIPS64 and using another define to choose that.
> 
> - TARGET_PPC64H, TARGET_PPCEMB : I see no reason why they cannot be
> handled dynamically as the other PowerPC CPU types, provided that
> TARGET_PPC64 is defined. Is it the long term plan ?

PowerPC embedded models are already available (should say should be as
none are actually implemented) when PPC64 is defined. But as those are
mainly PowerPC 32 with some extensions to manipulate the 64 bits GPR,
it's a great help if we can avoid doing all operations in 64 bits when
running on a 32 bits host (which would greatly decrease performances by
at least a factor of two, which is not acceptable). Then having a
specific 32 bits target using 64 bits register is very useful if one
want to use those features, but may be disabled if the host is 64 bits.
Note that most (all ?) embedded Freescale PowerPC microcontrollers
implement those extensions and that some ones are greatly interrested
with having an usable emulation avaible for those CPUs.

The PowerPC 64 with hypervisor mode support could be removed only if:
- the fact of emulating hypervisor feature do not slowdown the emulator
(which I greatly doubt, but there may be great surprises)
- someone provide an open-source hypervisor, compatible with the ones
used on real machines, that would allow at least Linux to be able to run
on a CPU with hypervisor mode available. Most 64 bits PowerPC, including
the 970 (ie G5) have the hypervisor mode support implemented. If the
hypervisor mode emulation is present, the OS won't be allowed to access
most SPR and some exceptions will need to have some specific handlers in
the hypervisor firmware. As I don't know such a software available, the
hypervisor mode can not be enabled for "standard" PowerPC 64 emulation;
or no-one will be able to actually use the emulator, except if using the
venerable but mostly undocumented (and nearly impossible to find on real
hardware) PowerPC 620 CPU.
Furthermore, running (or emulating) a SMP machine on a 64 bits PowerPC
with hypervisor features without hypervisor software is exactly
impossible.
Then I don't see how we can do without a separated target for hypervisor
features support.