linux-kernel - Re: [PATCH 02/24] xen/arm: hypercalls

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Message-ID: <1343380764.6812.108.camel@zakaz.uk.xensource.com>
Date:	Fri, 27 Jul 2012 10:19:24 +0100
From:	Ian Campbell <Ian.Campbell@...rix.com>
To:	Christopher Covington <cov@...eaurora.org>
CC:	Stefano Stabellini <Stefano.Stabellini@...citrix.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	"linaro-dev@...ts.linaro.org" <linaro-dev@...ts.linaro.org>,
	"arnd@...db.de" <arnd@...db.de>,
	"konrad.wilk@...cle.com" <konrad.wilk@...cle.com>,
	"catalin.marinas@....com" <catalin.marinas@....com>,
	"Tim (Xen.org)" <tim@....org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 02/24] xen/arm: hypercalls

On Thu, 2012-07-26 at 20:19 +0100, Christopher Covington wrote:
> Hi Stefano,
> 
> On 07/26/2012 11:33 AM, Stefano Stabellini wrote:
> > Use r12 to pass the hypercall number to the hypervisor.
> > 
> > We need a register to pass the hypercall number because we might not
> > know it at compile time and HVC only takes an immediate argument.
> 
> You're not going to JIT assemble the appropriate HVC instruction? Darn.

;-)

> How many call numbers are there, though? 8?

The maximum currently defined hypercall number is 55, although there are
some small gaps so there's actually more like 45 in total.

>  It seems like it'd be
> reasonable to take the approach that seems to be favored for MRC/MCR
> instructions, using a function containing switch statement that chooses
> between several inline assembly instructions based off an enum passed to
> the function. See for example arch_timer_reg_read in
> arch/arm/kernel/arch_timer.c.

I don't think it is feasible with this number of hypercalls, even
accepting that in many cases the number will be a constant so gcc can
likely optimise almost all of it away.

Is there something wrong with the r12 based approach?

Ian.

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