Re: [Xen-devel] Re: [GIT PULL] Xen APIC hooks (with io_apic_ops)

From: Gerd Hoffmann
Date: Tue May 26 2009 - 17:21:20 EST

On 05/26/09 14:46, George Dunlap wrote:
On Mon, May 25, 2009 at 5:10 AM, Ingo Molnar<mingo@xxxxxxx> wrote:
Note that this design problem has been created by Xen,
intentionally, and Xen is now suffering under those bad technical
choices made years ago. It's not Linux's problem.

I'd like to respecfully disagree with this.

Well. Xen *does* suffer from bad technical choices made years ago. I'm pretty sure Xen would look radically different when being rewritten from scratch today.

One reason is that Xen predates vt and svm. With that in mind some of the xen interface bits don't look *that* odd any more. Back then it did made sense to handle things that way. The ioapic hypercalls discussed in this thread belong into that group IMHO.

Another reason is that Xen wasn't "designed". Xen was "hacked up". As far I know there is no document which describes the overall design of the guest/xen ABI. Also there is no documentation (other than code) which describes all details of the guest/xen ABI. Simple reason: The ABI wasn't designed. It was hammered into shape until it worked. On x86. The guys who attempted (and failed) to port xen to ppc had alot of *ahem* fun with that stuff. For example: Passing guest virtual addresses in (some) hypercalls. Also direct paging mode is a very x86-ish and is the reason for a number of ia64-ifdefs in places where you don't expect them ...

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