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Subject: Re: [PATCH] arm64: write_sysreg asm illegal for aarch32
To: Robin Murphy <robin.murphy@arm.com>, linux-kernel@vger.kernel.org
Cc: Mark Rutland <mark.rutland@arm.com>,
        Christoffer Dall <cdall@linaro.org>,
        Stefan Traby <stefan@hello-penguin.com>,
        Suzuki K Poulose <suzuki.poulose@arm.com>,
        Marc Zyngier <marc.zyngier@arm.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will.deacon@arm.com>, Dave Martin <Dave.Martin@arm.com>,
        linux-arm-kernel@lists.infradead.org
References: <20171101170014.20931-1-salyzyn@android.com>
 <35cf2062-f27e-16e6-19b1-cae5200d7081@arm.com>
From: Mark Salyzyn <salyzyn@android.com>
Message-ID: <24cdf320-35e3-76f2-6e07-8b5548ac67a0@android.com>
Date: Wed, 1 Nov 2017 10:49:00 -0700
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On 11/01/2017 10:14 AM, Robin Murphy wrote:
> On 01/11/17 16:58, Mark Salyzyn wrote:
>> Cross compiling to aarch32 (for vdso32) using clang correctly
>> identifies that (the unused) write_sysreg inline asm directive is
>> illegal in that architectural context:
>>
>> arch/arm64/include/asm/arch_timer.h: error: invalid input constraint 'rZ' in asm
>>          write_sysreg(cntkctl, cntkctl_el1);
>>          ^
>> arch/arm64/include/asm/sysreg.h: note: expanded from macro 'write_sysreg'
>>                       : : "rZ" (__val));
>>                           ^
>>
>> GCC normally checks for correctness everywhere. But uniquely for
>> unused asm, will optimize out and suppress the error report.
> It sounds more like some paths are wrong in the compat vDSO build if
> it's pulling in this header in the first place - nothing in this file is
> relevant to AArch32.
>
> Robin.
>
And yet, when you CROSS_COMPILE_ARM32 a vdso32, you have no choice but 
to utilize the arm64 headers since they contain all the relevant kernel 
structures and environment.

asm/arch_timer.h (remember we are using arm instructions to access 
arch64 timers)

linux/time.h (really only for struct timespec())

asm/processor.h (eg: cpu_relax())

pull in a _lot_ of architectural related cruft that always somehow picks 
up asm/sysreg.h somewhere in the multitude of includes to fulfill some 
unused inline's needs.

-- Mark