Re: [Xen-devel] Re: [patch 13/26] Xen-paravirt_ops: Consistently wrap paravirt ops callsites to make them patchable

Rusty Russell
2007-03-18T00:45:55Z

On Fri, 2007-03-16 at 10:24 +0100, Ingo Molnar wrote:
> * Jeremy Fitzhardinge <jeremy@goop.org> wrote:
>
> > Wrap a set of interesting paravirt_ops calls in a wrapper which makes
> > the callsites available for patching. Unfortunately this is pretty
> > ugly because there's no way to get gcc to generate a function call,
> > but also wrap just the callsite itself with the necessary labels.
> >
> > This patch supports functions with 0-4 arguments, and either void or
> > returning a value. 64-bit arguments must be split into a pair of
> > 32-bit arguments (lower word first). Small structures are returned in
> > registers.
>
> ugh. This is beyond ugly! Why dont we just compile two images, one for
> Xen and one for native, do two passes to get those two images and
> 'merge' them into a single vmlinuz (so that we still have a 'single'
> kernel unit to deal with on the distro side). This way we avoid all this
> crazy, limited, fragile patchery business...

But with lguest, VMI and kvm I don't think that's a good idea.

For background: the current patching code is ugly too, but it only deals
with the 6 most common functions, so it's contained. This gets us
pretty close to !CONFIG_PARAVIRT performance. (But slowdown is still
measurable).

We get worse with Xen wanting to alter mkpte et al. By the time we
patch another 6 functions (each one slightly different), we get pretty
close to general coverage anyway, so it's clearer to do them all.

I think the question is, can we do it better than this? My previous
attempts (which lead to the current code) ranged from ugly to very ugly
8(

Rusty.