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Subject: Re: [PATCH v2 3/6] ARM: trusted_foundations: do not use naked
 function
To:     Stefan Agner <stefan@agner.ch>,
        Thierry Reding <treding@nvidia.com>,
        Stephen Warren <swarren@wwwdotorg.org>, linux@armlinux.org.uk
Cc:     Stephen Warren <swarren@nvidia.com>,
        Robin Murphy <robin.murphy@arm.com>, ard.biesheuvel@linaro.org,
        arnd@arndb.de, nicolas.pitre@linaro.org, marc.zyngier@arm.com,
        behanw@converseincode.com, keescook@chromium.org,
        Bernhard.Rosenkranzer@linaro.org, mka@chromium.org,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org
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From:   Dmitry Osipenko <digetx@gmail.com>
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Date:   Sun, 20 May 2018 01:02:46 +0300
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On 16.04.2018 21:21, Stefan Agner wrote:
> 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@gmail.com>
>>>>>>> Cc: Stephen Warren <swarren@nvidia.com>
>>>>>>> Cc: Thierry Reding <treding@nvidia.com>
>>>>>>> Signed-off-by: Stefan Agner <stefan@agner.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?

I've been preparing patches for upstream to add initial support of L2 cache
maintance to TF / Tegra30 and noticed that without this patch I'm getting a hang
early in boot. That is because before this patch registers store / restore was
incorrect, probably the premature return (lr -> pc) causes stack corruption. Not
sure whether it's worth to backport this patch, but I want to see it at least in
-next.

Thierry, please take care of this patch. Thanks.