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SubjectRe: [PATCH v2 3/6] ARM: trusted_foundations: do not use naked function
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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.

Robin.

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