Re: [PATCH 3/5] ARM: trusted_foundations: do not use naked function

[Stefan Agner <stefan@agner.ch>]

On 21.03.2018 18:16, Robin Murphy wrote:
> On 21/03/18 16:40, Stephen Warren wrote:
>> On 03/21/2018 09:26 AM, Dmitry Osipenko wrote:
>>> On 21.03.2018 17:09, Stefan Agner wrote:
>>>> On 21.03.2018 13:13, Robin Murphy wrote:
>>>>> On 20/03/18 23:02, 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.
>>>>>>
>>>>>> Additionally also make sure all callee-saved registers get saved
>>>>>> as it has been done before.
>>>>>>
>>>>>> Signed-off-by: Stefan Agner <stefan@agner.ch>
>>>>>> ---
>>>>>>    arch/arm/firmware/trusted_foundations.c | 12 +++++++-----
>>>>>>    1 file changed, 7 insertions(+), 5 deletions(-)
>>>>>>
>>>>>> diff --git a/arch/arm/firmware/trusted_foundations.c b/arch/arm/firmware/trusted_foundations.c
>>>>>> index 3fb1b5a1dce9..426d732e6591 100644
>>>>>> --- a/arch/arm/firmware/trusted_foundations.c
>>>>>> +++ b/arch/arm/firmware/trusted_foundations.c
>>>>>> @@ -31,21 +31,23 @@
>>>>>>      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"
>>>>>>            __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}"
>>>>>>            :
>>>>>> -        : "r" (type), "r" (arg1), "r" (arg2)
>>>>>> -        : "memory");
>>>>>> +        : "r" (r0), "r" (r1), "r" (r2)
>>>>>> +        : "memory", "r3", "r4", "r5", "r6", "r7", "r8", "r9", "r10");
>>>>>
>>>>> I may be missing a subtlety, but it looks like we no longer have a
>>>>> guarantee that r11 will be caller-saved as it was previously. I don't
>>>>> know the Trusted Foundations ABI to say whether that matters or not,
>>>>> but if it is the case that it never needed preserving anyway, that
>>>>> might be worth calling out in the commit message.
>>>>
>>>> Adding r11 (fp) to the clobber list causes an error when using gcc and
>>>> CONFIG_FRAME_POINTER=y:
>>>> arch/arm/firmware/trusted_foundations.c: In function ‘tf_generic_smc’:
>>>> arch/arm/firmware/trusted_foundations.c:51:1: error: fp cannot be used
>>>> in asm here
>>>>
>>>> Not sure what ABI Trusted Foundations follow.
>>>>
>>>> [adding Stephen, Thierry and Dmitry]
>>>> Maybe someone more familiar with NVIDIA Tegra SoCs can help?
>>>>
>>>> When CONFIG_FRAME_POINTER=y fp gets saved anyway. So we could add r11 to
>>>> clobber list ifndef CONFIG_FRAME_POINTER...
>>>
>>> I have no idea about TF ABI either. Looking at the downstream kernel code, r4 -
>>> r12 should be saved. I've CC'd Alexandre as he is the author of the original
>>> patch and may still remember the details.
>>>
>>> I'm also wondering why original code doesn't have r3 in the clobber list and why
>>> r3 is set to '0', downstream sets it to the address of SP and on return from SMC
>>> r3 contains the address of SP which should be restored. I'm now wondering how
>>> SMC calling worked for me at all on T30, maybe it didn't..
>>
>> I don't know what the ABI for ATF is. I assume it's documented in the ATF, PSCI, or similar specification, or ATF source code. Hence, I don't know whether ATF restores fp/r11.
> 
> Oops, I think we're starting to diverge here - "ATF" (as in "Arm
> Trusted Firmware") does implement the ARM SMCCC, which more or less
> just follows the regular procedure call standard in terms of register
> saving. The "TF" in question here is "Trusted Foundations" from
> Trusted Logic (who apparently don't exist any more) which is
> explicitly called out in the header as having its own nonstandard
> calling convention. I guess newer Tegras are using the former, whereas
> the older ones used the latter.
> 

What do you mean by "called out in the header as having its own
nonstandard"?

It is unclear what ABI is used, I just inferred from the fact that
register have been saved before that it might use a nonstandard calling
convention. 

Tegra 4i/TK1 and newer seem to use something called Trusted Little
Kernel.

>> My guess is that r3/r4 are set to 0 because they're defined as inputs by the SMC/ATF ABI, yet nothing the kernel does needed that many parameters, so they're hard-coded to 0 (to ensure they're set to something predictable) rather than also being parameters to tf_generic_smc().
>>
>> The original code used to save/restore a lot of registers, including r11/fp. Can't we side-step the issue of including/not-including r11/fp in the clobber list by not removing those stmfd/ldmfd assembly instructions?
> 
> That might be reasonable - fiddling with a C function's stack inside
> an asm is a bit grim, but for this case I can't see that it would mess
> with unwinding etc. or otherwise go wrong any more than the existing
> code, and I doubt the slight efficiency hit from having to change the 
> "pop the LR straight into the PC" idiom matters much.

Sounds reasonable, I guess in that case we can also omit all the
additional register in the clobber list.

--
Stefan