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

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Date:	Fri, 27 Jul 2012 08:56:06 -0400
From:	Christopher Covington <cov@...eaurora.org>
To:	Ian Campbell <Ian.Campbell@...rix.com>
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 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.
> 
> ;-)
> 
>> 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?

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.

Christopher

-- 
Employee of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum
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