Re: [Xen-devel] Re: [RFC, PATCH 5/24] i386 Vmi code patching

[Zachary Amsden <zach@vmware.com>]

Keir Fraser wrote:
>>
>> Yeah, point is the interface is normal C API, and has the similar free
>> form that normal kernel API's have.
>
> i think this sounds very sane, and an OS-specific interface shim gets 
> around problems such as finding CPU-specific state -- we can get at 
> smp_processor_id() just the same as the rest of the kernel, for 
> example. We could extend the concept of the interface shim we already 
> have -- a set of OS-specific high performance shims, plus a fallback 
> OS-agnostic shim.

Getting at smp_processor_id() is exactly the type of thing you _don't_ 
want to do.  You really can't have callbacks into the guest in the 
hypervisor platform layer.  It really is not efficient, and you cause 
yourself more trouble than it is worth.

And where exactly is smp_processor_id() exported to modules?  It's not.  
You've just locked your module into the current kernel's idea of how to 
get at smp_processor_id().  It changes based on compilation options of 
the kernel - for example, it is different with 4K stacks.  It has 
changed from a number of other different options in the past.

The fact that XenoLinux needs smp_processor_id() at all is quite 
ludicrous.  To disable interrupts, which is used fairly commonly to 
disable pre-emption as well, what does XenoLinux have to do?

It has to disable pre-emption to call smp_processor_id() so that it can 
disable interrupts, the re-enable preemption so that it can disable 
pre-emption.

That is truly convoluted, and is exactly why you should never get into 
these types of situations to begin with.

Zach