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

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Date:	Mon, 14 Sep 2015 10:30:30 +0200
From:	Jens Wiklander <jens.wiklander@...aro.org>
To:	Will Deacon <will.deacon@....com>
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 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:
> > > > > Adds helpers to do SMC based on ARM SMC Calling Convention.
> > > > > CONFIG_HAVE_SMCCC is enabled for architectures that may support
> > > > > the SMC instruction. It's the responsibility of the caller to
> > > > > know if the SMC instruction is supported by the platform.
> > > > 
> > > > [...]
> > > > > diff --git a/arch/arm64/kernel/smccc-call.S b/arch/arm64/kernel/smccc-call.S
> > > > > new file mode 100644
> > > > > index 0000000..3ce7fe8
> > > > > --- /dev/null
> > > > > +++ b/arch/arm64/kernel/smccc-call.S
> > > > > @@ -0,0 +1,34 @@
> > > > > +/*
> > > > > + * Copyright (c) 2015, Linaro Limited
> > > > > + *
> > > > > + * This program is free software; you can redistribute it and/or modify
> > > > > + * it under the terms of the GNU General Public License Version 2 as
> > > > > + * published by the Free Software Foundation.
> > > > > + *
> > > > > + * This program is distributed in the hope that it will be useful,
> > > > > + * but WITHOUT ANY WARRANTY; without even the implied warranty of
> > > > > + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
> > > > > + * GNU General Public License for more details.
> > > > > + *
> > > > > + */
> > > > > +#include <linux/linkage.h>
> > > > > +
> > > > > +#define SMC_PARAM_W0_OFFS      0
> > > > > +#define SMC_PARAM_W2_OFFS      8
> > > > > +#define SMC_PARAM_W4_OFFS      16
> > > > > +#define SMC_PARAM_W6_OFFS      24
> > > > > +
> > > > > +/* void smccc_call32(struct smccc_param32 *param) */
> > > > > +ENTRY(smccc_call32)
> > > > > +       stp     x28, x30, [sp, #-16]!
> > > > 
> > > > Why are you saving lr?
> > > 
> > > Agree, no point in saving lr, but I still need to decrease sp with 16 to
> > > maintain correct alignment. I'll do it with an str instruction instead.
> > 
> > That or pad out with xzr
> > 
> > > > 
> > > > > +       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?

Thanks,
Jens
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