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

[Rusty Russell <rusty@xxxxxxxxxxxxxxx>]

On Mon, 2007-03-19 at 11:38 -0700, Linus Torvalds wrote:
> 
> On Mon, 19 Mar 2007, Eric W. Biederman wrote:
> > 
> > True.  You can use all of the call clobbered registers.
> 
> Quite often, the biggest single win of inlining is not so much the code 
> size (although if done right, that will be smaller too), but the fact that 
> inlining DOES NOT CLOBBER AS MANY REGISTERS!

Thanks Linus.

*This* was the reason that the current hand-coded calls only clobber %
eax.  It was a compromise between native (no clobbers) and others (might
need a reg).

Now, since we decided to allow paravirt_ops operations to be normal C
(ie. the patching is optional and done late), we actually push and pop %
ecx and %edx.  This makes the call site 10 bytes long, which is a nice
size for patching anyway (enough for a movl $0, <addr>, a-la lguest's
cli, or movw $0, %gs:<addr> if we supported SMP).

The current 6 paravirt ops which are patched cover the vast majority of
calls (until the Xen patches, then we need ~4 more?).  Jeremy chose to
expand patching to cover *all* paravirt ops, rather than just the new
hot ones, and that's where we tipped over the ugliness threshold.

Cheers,
Rusty.


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