[PATCH 3/5] ARM: trusted_foundations: do not use naked function
Dmitry Osipenko
digetx at gmail.com
Thu Mar 22 07:03:02 PDT 2018
On 22.03.2018 15:43, Stefan Agner wrote:
> On 22.03.2018 12:48, Robin Murphy wrote:
>> On 21/03/18 21:41, Stefan Agner wrote:
>>> 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 at 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"?
>>
>> Specifically, the comment in
>> arch/arm/include/asm/trusted_foundations.h which says:
>>
>> "The calls are completely specific to Trusted Foundations, and do
>> *not* follow the SMC calling convention or the PSCI standard."
>>
>
> Oh didn't notice that. Thanks for pointing out.
>
>>> 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.
>>
>> Yeah, you should only need to specify clobbers for any registers which
>> are neither used as arguments nor explicitly preserved - looking at
>> the layout of the code, it seems unlikely that the compiler would have
>> anything live in r3 or r12 across the call (since the scope for
>> inlining is pretty trivial), but there's no harm in being strictly
>> correct :)
>
> So something like this?
>
> -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");
: "memory", "r3", "r12", "lr");
> }
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