Re: [PATCH v2 2/8] arm64: Convert hcalls to use ISS field

From: Christopher Covington <cov@codeaurora.org>
To: Geoff Levand <geoff@infradead.org>
Cc: marc.zyngier@arm.com, Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will.deacon@arm.com>,
	linux-arm-kernel@lists.infradead.org,
	Grant Likely <grant.likely@linaro.org>,
	kexec@lists.infradead.org,
	Christoffer Dall <christoffer.dall@linaro.org>
Subject: Re: [PATCH v2 2/8] arm64: Convert hcalls to use ISS field
Date: Tue, 03 Mar 2015 16:47:32 -0500	[thread overview]
Message-ID: <54F62BF4.1070306@codeaurora.org> (raw)
In-Reply-To: <1425338536.9265.2.camel@infradead.org>

Hi Geoff,

On 03/02/2015 06:22 PM, Geoff Levand wrote:
> Hi Christoffer,
> 
> On Mon, 2015-03-02 at 14:13 -0800, Christoffer Dall wrote:
>> On Wed, Feb 25, 2015 at 02:09:30PM -0800, Geoff Levand wrote:
>>> The current hyp-stub vector implementation, which uses x0, is limited
>>> to two hyper calls; __hyp_get_vectors and __hyp_set_vectors.  To
>>> support cpu_soft_restart() we need a third hyper call, one which
>>> allows for code to be executed at EL2.  My proposed use of the
>>> immediate value of the hvc instruction will allow for 2^16 distinct
>>> hyper calls.  
>>
>> right, but using x0 allows for 2^64 distinct hypercalls.  Just to be
>> clear, I'm fine with using immediate field if there are no good reasons
>> not to, I was just curious as to what direct benefit it has.  After
>> thinking about it a bit, from my point of view, the benefit would be the
>> clarity that x0 is first argument like a normal procedure call, so no
>> need to shift things around.  Is this part of the equation or am I
>> missing the overall purpose here?
> 
> Yes, in general it will make marshaling of args, etc. easier.  Also,
> to me, if we are going to change the implementation it seems to be
> the most natural way.

From reading the architecture documentation, I too expected the hypervisor
call instruction's immediate and the instruction specific syndrome to be used.
However I vaguely recall someone pointing out that reading the exception
syndrome register and extracting the instruction specific syndrome is bound to
take longer than simply using a general purpose register.

One might also consider alignment with the SMC Calling Convention document
[1], which while originally written for SMC, is also used for HVC by PSCI [2].

1. http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.den0028a/index.html
2. http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.den0022c/index.html

Chris

-- 
Qualcomm Innovation Center, Inc.
The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum,
a Linux Foundation Collaborative Project

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