Re: [Xen-devel] Re: [RFC] [PATCH] sysfs support for Xen attributes

[Anthony Liguori <aliguori@xxxxxxxxxx>]

Dave Hansen wrote:

> The ppc64 hypervisor does something like this today in a couple of
> places.  It is kinda a mess.  I think that putting a generic, binary
> firmware interface leads to having a bit of a crutch.  It basically lets
> the userspace software stack bypass Linux and talk directly to the
> hypervisor.  It also means that you have to have a very specialized
> software stack for each hypervisor or virtualization type, which is very
> bad.
>
> This pushes things out to userspace, which is generally good.  But, it
> is pushing behavior and "hardware" knowledge out there, too.  The
> hardware knowledge, especially, is something that we usually try to
> encapsulate.
>  
The Xen virtual hardware is exposed in the normal way (there is a Xen 
bus so Xen devices show up under that).
> Also things like inter-partition page sharing, and partition migration
> are used in other hypervisors.  I think it is essential to get common
> interfaces to those things.
>  
In very, very different ways though.

> One last thing...  When you say "very strongly binary" do you mean, "are
> implemented now as very strongly binary", or "absolutely 100% have to be
> horribly strongly binary"?  They are two quite different things. :)
>  
To expose the hypercalls to userspace via sysfs (or another high level 
interface) would require a whole bunch of complex code to encode the 
hypercalls and decode there results.  I'm not sure having a common 
interface is a compelling argument to justify this kernel-level 
complexity since one can just standardize on a userspace library 
(something like http://www.libvir.org).
I do agree we need a common interface though...

Regards,

Anthony Liguori

> -- Dave
>
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