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

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Date:	Tue, 28 Feb 2012 10:12:50 +0000
From:	Ian Campbell <Ian.Campbell@...rix.com>
To:	Peter Maydell <peter.maydell@...aro.org>
CC:	Stefano Stabellini <Stefano.Stabellini@...citrix.com>,
	"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>,
	"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 Mon, 2012-02-27 at 21:05 +0000, Peter Maydell wrote:
> On 27 February 2012 16:27, Ian Campbell <Ian.Campbell@...rix.com> wrote:
> > R12 is not accessible from the 16 bit "T1" Thumb encoding of mov
> > immediate (which can only target r0..r7).
> >
> > Since we support only ARMv7+ there are "T2" and "T3" encodings available
> > which do allow direct mov of an immediate into R12, but are 32 bit Thumb
> > instructions.
> >
> > Should we use r7 instead to maximise instruction density for Thumb code?
> 
> r7 is (used by gcc as) the Thumb frame pointer; I don't know if this
> makes it worth avoiding in this context.

I think it does.

It actually sounds as if using r12 is fine here, the impact on code
density should be pretty small -- there aren't really all that many call
sites which involve hypercalls.

By way of an example I measured an x86 kernel which should be using more
hypercalls due to pv paging etc and found that 0.014% of the lines in
"objdump -d" contained a call to the hypercall_page. (I know not all
lines of objdump -d output are instructions but it's a reasonable approx
IMHO).

So I think using 3 16 bit instructions slots instead of 2 won't make
much impact in practice.

Thanks,
Ian.

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