[PATCH-WIP 01/13] xen/arm: use r12 to pass the hypercall number to the hypervisor

[Stefano Stabellini]

On Thu, 1 Mar 2012, Russell King - ARM Linux wrote:
> On Thu, Mar 01, 2012 at 10:27:02AM +0000, Dave Martin wrote:
> > So, where there's a compelling reason to inline these things, we can use
> > the existing techniques if we're alert to the risks.  But in cases where
> > there isn't a compelling reason, aren't we just inviting fragility
> > unnecessarily?
> 
> The practical experience from the kernel suggests that there isn't a
> problem - that's not to say that future versions of gcc won't become
> a problem, and that the compiler guys may refuse to fix it.
> 
> I think it's a feature that we should be pressing gcc guys for - it's
> fairly fundamental to any programming which requires interfaces that
> require certain args in certain registers, or receive results in
> certain registers.
> 
> The options over this are basically:
> 1. refusing to upgrade to any version of gcc which does not allow
>    registers-in-asm
> 2. doing the store-to-memory reload-in-asm thing
> 3. hand-coding veneers for every call to marshall the registers
> 
> Each of those has its down sides, but I suspect with (1), it may be
> possible to have enough people applying pressure to the compiler guys
> that they finally see sense on this matter.

I tend to have a very practical approach about this sort of issues, so I
am tempted to go with 1) if you agree.