linux-kernel - Re: arm64 -- psci tell the compiler in which registers we are expecting arguments

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Message-ID: <20150123175622.GD13772@arm.com>
Date:	Fri, 23 Jan 2015 17:56:22 +0000
From:	Will Deacon <will.deacon@....com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	Andy Whitcroft <apw@...onical.com>,
	Catalin Marinas <Catalin.Marinas@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: arm64 -- psci tell the compiler in which registers we are
 expecting arguments

On Fri, Jan 23, 2015 at 05:54:45PM +0000, Russell King - ARM Linux wrote:
> On Fri, Jan 23, 2015 at 05:50:39PM +0000, Will Deacon wrote:
> > On Fri, Jan 23, 2015 at 05:31:54PM +0000, Andy Whitcroft wrote:
> > > diff --git a/arch/arm64/kernel/psci.c b/arch/arm64/kernel/psci.c
> > > index f1dbca7..425cf78 100644
> > > --- a/arch/arm64/kernel/psci.c
> > > +++ b/arch/arm64/kernel/psci.c
> > > @@ -113,9 +113,14 @@ static void psci_power_state_unpack(u32 power_state,
> > >   * The following two functions are invoked via the invoke_psci_fn pointer
> > >   * and will not be inlined, allowing us to piggyback on the AAPCS.
> > >   */
> > > -static noinline int __invoke_psci_fn_hvc(u64 function_id, u64 arg0, u64 arg1,
> > > -					 u64 arg2)
> > > +static noinline int __invoke_psci_fn_hvc(u64 _function_id, u64 _arg0,
> > > +					 u64 _arg1, u64 _arg2)
> > >  {
> > > +	register u64 function_id asm("x0") = _function_id;
> > > +	register u64 arg0 asm("x1") = _arg0;
> > > +	register u64 arg1 asm("x2") = _arg1;
> > > +	register u64 arg2 asm("x3") = _arg2;
> > > +
> > 
> > Given that we've already been bitten by the compiler, I think we should
> > just bite the bullet and implement these two functions in assembly in a
> > separate file rather than rely on register variables doing what we want.
> 
> The alternative is a naked function.

I wondered about that, but the gcc docs say that you can't then provide
operands to the asm block, which would mean removing the __asmeq checks
altogether (which terrifies me!).

Will
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