Re: [Xen-devel] Genapic

[Andi Kleen <ak@xxxxxx>]

On Wed, May 18, 2005 at 02:45:49PM -0500, Natasha Jarymowycz wrote:
> Based upon the discussion so far, I think this is where xen stands:
> 
> Xen is currently based upon linux-i386 code, including support for
> mach-specific functionality in asm-x86/mach-xxx subdirs.
> 
> Xen is responsible for initializing local APICs and so it must include
> support for these machines (it can't be deferred to linux).
> 
> Linux (dom 0) may or may not need to include support for these machines
> as well depending upon other features (timers? numa?) which may be
> added in the future.  This is not an issue for Xen 3.0
> 
> Adding genapic support to Xen a la linux-i386 should be trivial given
> the current state of the code.  [Genapic allows the appropriate mach-xxx
> code to be chosen during run time.  This is considered very important
> from a distro stand point]
> 
> Linux-x86-64 code is very different from i386 from a code organization
> standpoint.  Using apic as an example, it chooses between flat or
> clustered rather than es7000 vs summit vs xxx.

The behaviour is basically the same. The reason i386 is so different
has more historical reasons and that it supports subarchitectures
for non PC like x86 systems that need much more changes (like NUMAQ
or pc98). genapic actually doesnt support these, but the i386
subarchitectures had to be still fit into that framework.

I doubt Xen will ever run on any of these.

On x86-64 only the APIC differences are abstracted this way,
which is enough for all PC Like systems (including Summit and ES7000) 

For Xen I would recommend to go with the simpler model from x86-64.

> 
> sub-arch machines.  Given the impending move to Xen 3.0, I think it would
> be best to pull in the needed Linux-i386 genapic code near-term.
> This code may very well not be sufficient for newer x86-64 machines

It should, but the x86-64 code is much cleaner and should
be sufficient.

> Personally, I don't know.  I also don't know if the x86-64 code can be
> easily ported to work on the i386 machines currently being targeted.

It should.

-Andi

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