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Date:   Mon, 16 Apr 2018 20:21:09 +0200
From:   Stefan Agner <stefan@...er.ch>
To:     Thierry Reding <treding@...dia.com>,
        Stephen Warren <swarren@...dotorg.org>, linux@...linux.org.uk
Cc:     Stephen Warren <swarren@...dia.com>,
        Robin Murphy <robin.murphy@....com>, ard.biesheuvel@...aro.org,
        arnd@...db.de, nicolas.pitre@...aro.org, marc.zyngier@....com,
        behanw@...verseincode.com, keescook@...omium.org,
        Bernhard.Rosenkranzer@...aro.org, mka@...omium.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Dmitry Osipenko <digetx@...il.com>
Subject: Re: [PATCH v2 3/6] ARM: trusted_foundations: do not use naked
 function

On 16.04.2018 18:08, Stephen Warren wrote:
> On 04/16/2018 09:56 AM, Stefan Agner wrote:
>> On 27.03.2018 14:16, Dmitry Osipenko wrote:
>>> On 27.03.2018 14:54, Robin Murphy wrote:
>>>> On 26/03/18 22:20, Dmitry Osipenko wrote:
>>>>> On 25.03.2018 21:09, Stefan Agner wrote:
>>>>>> As documented in GCC naked functions should only use Basic asm
>>>>>> syntax. The Extended asm or mixture of Basic asm and "C" code is
>>>>>> not guaranteed. Currently this works because it was hard coded
>>>>>> to follow and check GCC behavior for arguments and register
>>>>>> placement.
>>>>>>
>>>>>> Furthermore with clang using parameters in Extended asm in a
>>>>>> naked function is not supported:
>>>>>>     arch/arm/firmware/trusted_foundations.c:47:10: error: parameter
>>>>>>             references not allowed in naked functions
>>>>>>                   : "r" (type), "r" (arg1), "r" (arg2)
>>>>>>                          ^
>>>>>>
>>>>>> Use a regular function to be more portable. This aligns also with
>>>>>> the other smc call implementations e.g. in qcom_scm-32.c and
>>>>>> bcm_kona_smc.c.
>>>>>>
>>>>>> Cc: Dmitry Osipenko <digetx@...il.com>
>>>>>> Cc: Stephen Warren <swarren@...dia.com>
>>>>>> Cc: Thierry Reding <treding@...dia.com>
>>>>>> Signed-off-by: Stefan Agner <stefan@...er.ch>
>>>>>> ---
>>>>>> Changes in v2:
>>>>>> - Keep stmfd/ldmfd to avoid potential ABI issues
>>>>>>
>>>>>>    arch/arm/firmware/trusted_foundations.c | 14 +++++++++-----
>>>>>>    1 file changed, 9 insertions(+), 5 deletions(-)
>>>>>>
>>>>>> diff --git a/arch/arm/firmware/trusted_foundations.c
>>>>>> b/arch/arm/firmware/trusted_foundations.c
>>>>>> index 3fb1b5a1dce9..689e6565abfc 100644
>>>>>> --- a/arch/arm/firmware/trusted_foundations.c
>>>>>> +++ b/arch/arm/firmware/trusted_foundations.c
>>>>>> @@ -31,21 +31,25 @@
>>>>>>      static unsigned long cpu_boot_addr;
>>>>>>    -static void __naked tf_generic_smc(u32 type, u32 arg1, u32 arg2)
>>>>>> +static void tf_generic_smc(u32 type, u32 arg1, u32 arg2)
>>>>>>    {
>>>>>> +    register u32 r0 asm("r0") = type;
>>>>>> +    register u32 r1 asm("r1") = arg1;
>>>>>> +    register u32 r2 asm("r2") = arg2;
>>>>>> +
>>>>>>        asm volatile(
>>>>>>            ".arch_extension    sec\n\t"
>>>>>> -        "stmfd    sp!, {r4 - r11, lr}\n\t"
>>>>>> +        "stmfd    sp!, {r4 - r11}\n\t"
>>>>>>            __asmeq("%0", "r0")
>>>>>>            __asmeq("%1", "r1")
>>>>>>            __asmeq("%2", "r2")
>>>>>>            "mov    r3, #0\n\t"
>>>>>>            "mov    r4, #0\n\t"
>>>>>>            "smc    #0\n\t"
>>>>>> -        "ldmfd    sp!, {r4 - r11, pc}"
>>>>>> +        "ldmfd    sp!, {r4 - r11}\n\t"
>>>>>>            :
>>>>>> -        : "r" (type), "r" (arg1), "r" (arg2)
>>>>>> -        : "memory");
>>>>>> +        : "r" (r0), "r" (r1), "r" (r2)
>>>>>> +        : "memory", "r3", "r12", "lr");
>>>>>
>>>>> Although seems "lr" won't be affected by SMC invocation because it should be
>>>>> banked and hence could be omitted entirely from the code. Maybe somebody could
>>>>> confirm this.
>>>> Strictly per the letter of the architecture, the SMC could be trapped to Hyp
>>>> mode, and a hypervisor might clobber LR_usr in the process of forwarding the
>>>> call to the firmware secure monitor (since Hyp doesn't have a banked LR of its
>>>> own). Admittedly there are probably no real systems with the appropriate
>>>> hardware/software combination to hit that, but on the other hand if this gets
>>>> inlined where the compiler has already created a stack frame then an LR clobber
>>>> is essentially free, so I reckon we're better off keeping it for reassurance.
>>>> This isn't exactly a critical fast path anyway.
>>>
>>> Okay, thank you for the clarification.
>>
>> So it seems this change is fine?
>>
>> Stephen, you picked up changes for this driver before, is this patch
>> going through your tree?
> 
> You had best ask Thierry; he's taken over Tegra maintenance upstream.
> But that said, don't files in arch/arm go through Russell?

I think the last patches applied to that file went through your tree.

Thierry, Russel, any preferences?

--
Stefan

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