linux-kernel - Re: [PATCH v5 1/5] arm/arm64: add smccc ARCH32

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Message-ID: <20150916171831.GS28771@arm.com>
Date:	Wed, 16 Sep 2015 18:18:31 +0100
From:	Will Deacon <will.deacon@....com>
To:	Jens Wiklander <jens.wiklander@...aro.org>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	Arnd Bergmann <arnd@...db.de>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Kees Cook <keescook@...omium.org>,
	"valentin.manea@...wei.com" <valentin.manea@...wei.com>,
	Mark Rutland <Mark.Rutland@....com>,
	"javier@...igon.com" <javier@...igon.com>,
	"emmanuel.michel@...com" <emmanuel.michel@...com>,
	Michal Simek <michal.simek@...inx.com>,
	"jean-michel.delorme@...com" <jean-michel.delorme@...com>,
	Jason Gunthorpe <jgunthorpe@...idianresearch.com>,
	Rob Herring <robh+dt@...nel.org>
Subject: Re: [PATCH v5 1/5] arm/arm64: add smccc ARCH32

On Tue, Sep 15, 2015 at 10:05:59PM +0100, Jens Wiklander wrote:
> On Tue, Sep 15, 2015 at 07:26:45PM +0100, Will Deacon wrote:
> > On Mon, Sep 14, 2015 at 09:30:30AM +0100, Jens Wiklander wrote:
> > > On Fri, Aug 21, 2015 at 01:43:31PM +0200, Jens Wiklander wrote:
> > > > On Fri, Aug 21, 2015 at 10:24:30AM +0100, Will Deacon wrote:
> > > > > On Thu, Aug 20, 2015 at 12:37:29PM +0100, Jens Wiklander wrote:
> > > > > > On Wed, Aug 19, 2015 at 05:50:09PM +0100, Will Deacon wrote:
> > > > > > > On Wed, Aug 19, 2015 at 09:40:25AM +0100, Jens Wiklander wrote:
> > > > > > > > +       mov     x28, x0
> > > > > > > > +       ldp     w0, w1, [x28, #SMC_PARAM_W0_OFFS]
> > > > > > > > +       ldp     w2, w3, [x28, #SMC_PARAM_W2_OFFS]
> > > > > > > > +       ldp     w4, w5, [x28, #SMC_PARAM_W4_OFFS]
> > > > > > > > +       ldp     w6, w7, [x28, #SMC_PARAM_W6_OFFS]
> > > > > > > > +       smc     #0
> > > > > > > > +       stp     w0, w1, [x28, #SMC_PARAM_W0_OFFS]
> > > > > > > > +       stp     w2, w3, [x28, #SMC_PARAM_W2_OFFS]
> > > > > > > > +       ldp     x28, x30, [sp], #16
> > > > > > > > +       ret
> > > > > > > > +ENDPROC(smccc_call32)
> > > > > > > 
> > > > > > > Could we deal with this like we do for PSCI instead? (see
> > > > > > > __invoke_psci_fn_smc). We could also then rename psci-call.S to fw-call.S
> > > > > > > and stick this in there too.
> > > > > > 
> > > > > > I assume you're referring to when to use "hvc" and "smc".
> > > > > 
> > > > > No, I mean use a C prototype to avoid marshalling the parameters in assembly
> > > > > like this. As Rutland pointed out, the return value is a bit messy, but
> > > > > the arguments align nicely with the PCS afaict.
> > > > 
> > > > If possible I'd like the function to have the same prototype for both
> > > > arm and arm64. For arm it's not possible to supply more than 4
> > > > parameters. To fully support SMC Calling Convention we need to be able
> > > > to pass 8 parameters and have 4 return values. The OP-TEE driver in this
> > > > patch set depends on this. I don't see how we can avoid the marshalling
> > > > here.
> > > > 
> > > > We could have two versions of the SMCCC functions, one simplified which
> > > > only uses registers and one complete like this one with marshalling.
> > > 
> > > Will, what do think about this?
> > 
> > I still think you should make use of a C prototype to avoid explicit
> > parameter marshalling in assembly. If you want to maintain a compatible
> > API between arm and arm64, then you can easily have an intermediate
> > function in arm64 that sits between the API entry point and the assembly.
> 
> Yes, I see how that's convenient for passing argument values in
> registers, but that doesn't help with storing the returned values in
> x0..x3 into something accessible in C. Am I missing something?

Your approach using a struct looked fine to me, just place it after the
parameters and use it only for the return values. Then __invoke_psci*
can wrap this and unpack r0/x0 from the structure.

Will
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