linux-kernel - Re: [PATCH v2 15/16] arm/arm64: smccc: Implement SMCCC v1.1 inline primitive

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Date:   Tue, 30 Jan 2018 12:27:23 +0000
From:   Marc Zyngier <marc.zyngier@....com>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        kvmarm <kvmarm@...ts.cs.columbia.edu>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Peter Maydell <peter.maydell@...aro.org>,
        Christoffer Dall <christoffer.dall@...aro.org>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Mark Rutland <mark.rutland@....com>,
        Robin Murphy <robin.murphy@....com>,
        Jon Masters <jcm@...hat.com>
Subject: Re: [PATCH v2 15/16] arm/arm64: smccc: Implement SMCCC v1.1 inline
 primitive

On 29/01/18 21:45, Ard Biesheuvel wrote:
> On 29 January 2018 at 17:45, Marc Zyngier <marc.zyngier@....com> wrote:
>> One of the major improvement of SMCCC v1.1 is that it only clobbers
>> the first 4 registers, both on 32 and 64bit. This means that it
>> becomes very easy to provide an inline version of the SMC call
>> primitive, and avoid performing a function call to stash the
>> registers that would otherwise be clobbered by SMCCC v1.0.
>>
>> Signed-off-by: Marc Zyngier <marc.zyngier@....com>
>> ---
>>  include/linux/arm-smccc.h | 157 ++++++++++++++++++++++++++++++++++++++++++++++
>>  1 file changed, 157 insertions(+)
>>
>> diff --git a/include/linux/arm-smccc.h b/include/linux/arm-smccc.h
>> index dd44d8458c04..bc5843728909 100644
>> --- a/include/linux/arm-smccc.h
>> +++ b/include/linux/arm-smccc.h
>> @@ -150,5 +150,162 @@ asmlinkage void __arm_smccc_hvc(unsigned long a0, unsigned long a1,
>>
>>  #define arm_smccc_hvc_quirk(...) __arm_smccc_hvc(__VA_ARGS__)
>>
>> +/* SMCCC v1.1 implementation madness follows */
>> +#ifdef CONFIG_ARM64
>> +
>> +#define SMCCC_SMC_INST "smc    #0"
>> +#define SMCCC_HVC_INST "hvc    #0"
>> +
>> +#define __arm_smccc_1_1_prologue(inst)                                 \
>> +       inst "\n"                                                       \
>> +       "cbz    %[ptr], 1f\n"                                           \
>> +       "stp    %x[r0], %x[r1], %[ra0]\n"                               \
>> +       "stp    %x[r2], %x[r3], %[ra2]\n"                               \
>> +       "1:\n"                                                          \
>> +       : [ra0] "=Ump" (*(&___res->a0)),                                \
>> +         [ra2] "=Ump" (*(&___res->a2)),
>> +
>> +#define __arm_smccc_1_1_epilogue       : "memory"
>> +
>> +#endif
>> +
>> +#ifdef CONFIG_ARM
>> +#include <asm/opcodes-sec.h>
>> +#include <asm/opcodes-virt.h>
>> +
>> +#define SMCCC_SMC_INST __SMC(0)
>> +#define SMCCC_HVC_INST __HVC(0)
>> +
>> +#define __arm_smccc_1_1_prologue(inst)                                 \
>> +       inst "\n"                                                       \
>> +       "cmp    %[ptr], #0\n"                                           \
>> +       "stmne  %[ptr], {%[r0], %[r1], %[r2], %[r3]}\n"                 \
>> +       : "=m" (*___res),
>> +
>> +#define __arm_smccc_1_1_epilogue       : "memory", "cc"
>> +
>> +#endif
>> +
>> +#define __constraint_write_0                                           \
>> +       [r0] "+r" (r0), [r1] "=r" (r1), [r2] "=r" (r2), [r3] "=r" (r3)
>> +#define __constraint_write_1                                           \
>> +       [r0] "+r" (r0), [r1] "+r" (r1), [r2] "=r" (r2), [r3] "=r" (r3)
>> +#define __constraint_write_2                                           \
>> +       [r0] "+r" (r0), [r1] "+r" (r1), [r2] "+r" (r2), [r3] "=r" (r3)
>> +#define __constraint_write_3                                           \
>> +       [r0] "+r" (r0), [r1] "+r" (r1), [r2] "+r" (r2), [r3] "+r" (r3)
> 
> It seems you need +r for all arguments, otherwise the compiler will
> notice that the value is never used, and may assign the register to
> 'res' instead, i.e.,
> 
>  3e4:   320107e0        mov     w0, #0x80000001                 // #-2147483647
>  3e8:   320183e1        mov     w1, #0x80008000                 // #-2147450880
>  3ec:   910123a2        add     x2, x29, #0x48
>  3f0:   d4000002        hvc     #0x0
>  3f4:   b4000062        cbz     x2, 400 <enable_psci_bp_hardening+0x88>
>  3f8:   a90487a0        stp     x0, x1, [x29, #72]
>  3fc:   a9058fa2        stp     x2, x3, [x29, #88]
> 
> (for the code generated in the next patch)

Well spotted.

I think this is because of the lack of early-clobber for the unassigned
registers. The compiler assumes the whole sequence is a single
instruction, with the output registers being affected at the end. If we
mark those with '=&r', we will prevent GCC from emitting this kind of
horror.

Note that with Robin's trick of moving the assignment back to C code,
this is a bit moot as this is really a single instruction (smc/hvc), and
there is no intermediate register evaluation.

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...