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

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Date:	Fri, 23 Jan 2015 17:50:39 +0000
From:	Will Deacon <will.deacon@....com>
To:	Andy Whitcroft <apw@...onical.com>
Cc:	Catalin Marinas <Catalin.Marinas@....com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: arm64 -- psci tell the compiler in which registers we are
 expecting arguments

Hi Andy,

Thanks for the patch.

On Fri, Jan 23, 2015 at 05:31:54PM +0000, Andy Whitcroft wrote:
> When compiling the kernel with very recent gcc-4.9 compilers we see the
> following errors:
> 
>     /tmp/ccEayz7S.s: Assembler messages:
>     /tmp/ccEayz7S.s:41: Error: .err encountered
>     /tmp/ccEayz7S.s:42: Error: .err encountered
>     /tmp/ccEayz7S.s:43: Error: .err encountered
>     /tmp/ccEayz7S.s:101: Error: .err encountered
>     /tmp/ccEayz7S.s:102: Error: .err encountered
>     /tmp/ccEayz7S.s:103: Error: .err encountered
> 
> This is due to assumptions about the register assignments made by gcc on
> function entry.  The errors above occur when these assignments are not
> as expected.  It turns out that when you enable mcount preables (-pg)
> these are altered and the compilation failed.
> 
> Inform the compiler of these register requirements.
> 
> BugLink: http://bugs.launchpad.net/bugs/1414002
> Signed-off-by: Andy Whitcroft <apw@...onical.com>
> ---
>  arch/arm64/kernel/psci.c | 18 ++++++++++++++----
>  1 file changed, 14 insertions(+), 4 deletions(-)
> 
> 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.

Will
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