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

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Message-ID: <alpine.DEB.2.02.1207271454470.26163@kaball.uk.xensource.com>
Date:	Fri, 27 Jul 2012 14:57:52 +0100
From:	Stefano Stabellini <stefano.stabellini@...citrix.com>
To:	Christopher Covington <cov@...eaurora.org>
CC:	Ian Campbell <Ian.Campbell@...rix.com>,
	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 Fri, 27 Jul 2012, Christopher Covington wrote:
> On 07/27/2012 05:19 AM, Ian Campbell wrote:
> > 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.
> > 
> > ;-)
> > 

I admit having spent few hours thinking about how to implement a
self-modifying function able to change the ISS at run time. Fortunately
few hours later I was struck by common sense and I decided to follow a
different direction ;-)


> > 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?
> 
> Only that you're defining a custom interface for something that there is
> a potentially more standard interface for. I just wanted to double check
> that all the ways of using the potentially more standard interface had
> been explored and found to be unreasonable.

Yep, thanks for helping us reviewing the code.
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