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

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Date:	Sat, 17 Mar 2007 20:26:29 +1100
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Jeremy Fitzhardinge <jeremy@...p.org>,
	Zachary Amsden <zach@...are.com>,
	xen-devel@...ts.xensource.com, virtualization@...ts.osdl.org,
	linux-kernel@...r.kernel.org, Chris Wright <chrisw@...s-sol.org>,
	Andi Kleen <ak@....de>,
	Anthony Liguori <anthony@...emonkey.ws>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [Xen-devel] Re: [patch 13/26] Xen-paravirt_ops: Consistently
	wrap paravirt ops callsites to make them patchable

On Fri, 2007-03-16 at 10:24 +0100, Ingo Molnar wrote:
> * Jeremy Fitzhardinge <jeremy@...p.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.

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