Re: [PATCH 00/23] Add KVM support for PPC64 (970) hosts

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

On Tue, 2009-07-07 at 17:44 +0200, Arnd Bergmann wrote:
> Most of the code is after 970 but does look generic enough to be
> usable
> on other powerpc64 implementations. How specific to 970 is it really?
> Maybe the files and identifiers could be renamed to ppc64?
> 
> You mentioned before that you could not get ppc32 guests to run on
> Cell hosts, but how about 970 or cell guests? Are there any problems
> that mean it cannot work on Power5/6/7 hosts, or was that just a
> matter of your priorities and available test hardware?

There is another problem I foresee which is the cache line size
difference.... That can not be emulated.

The cache line sizes vary between processors, that is, the problem is
not only classic 32 "32 bytes" vs. classic 64 "128 bytes", some CPUs
have 64 bytes cache line size for example too.

However, we can somewhat work around. At least 64-bit linux uses the
cache line size from the device-tree for example and overrides the
cputable content with that.

So if we manage to pass a "manufactures" device-tree down, we can cope
with the differences to some extent.

Things are going to be harder if the goal is to run some other OS, of
course, to the extent that we may not be able to sort it out completely
but at least for the linux-in-linux case it should work. For example, I
think MacOS uses the PVR and ignores the device-tree values...

Unfortunately, 32-bit linux does the same, though there is some work in
progress to change that, so it won't be a problem in the long run.

The patching technique Alexander does on executable pages is fun :-)
Though it's also both a bit slow and potentially dangerous (you don't
know for sure that what you are patching is actually an instruction).

Maybe it should be a configuration option to only be enabled when
needed.

Cheers,
Ben.