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

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Date:	Thu, 1 Mar 2012 10:35:20 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Dave Martin <dave.martin@...aro.org>
Cc:	Ian Campbell <Ian.Campbell@...rix.com>,
	Peter Maydell <peter.maydell@...aro.org>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	"linaro-dev@...ts.linaro.org" <linaro-dev@...ts.linaro.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"arnd@...db.de" <arnd@...db.de>,
	Stefano Stabellini <Stefano.Stabellini@...citrix.com>,
	"catalin.marinas@....com" <catalin.marinas@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	David Vrabel <david.vrabel@...rix.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH-WIP 01/13] xen/arm: use r12 to pass the hypercall
	number to the hypervisor

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.
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