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

[Dmitry Osipenko]

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:
>>> ÂÂ arch/arm/firmware/trusted_foundations.c:47:10: error: parameter
>>> ÂÂÂÂÂÂÂÂÂÂ references not allowed in naked functions
>>> ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ : "r" (type), "r" (arg1), "r" (arg2)
>>> ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ ^
>>>
>>> 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@xxxxxxxxx>
>>> Cc: Stephen Warren <swarren@xxxxxxxxxx>
>>> Cc: Thierry Reding <treding@xxxxxxxxxx>
>>> Signed-off-by: Stefan Agner <stefan@xxxxxxxx>
>>> ---
>>> Changes in v2:
>>> - Keep stmfd/ldmfd to avoid potential ABI issues
>>>
>>> Â arch/arm/firmware/trusted_foundations.c | 14 +++++++++-----
>>> Â 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 @@
>>> Â Â static unsigned long cpu_boot_addr;
>>> Â -static void __naked tf_generic_smc(u32 type, u32 arg1, u32 arg2)
>>> +static void tf_generic_smc(u32 type, u32 arg1, u32 arg2)
>>> Â {
>>> +ÂÂÂ register u32 r0 asm("r0") = type;
>>> +ÂÂÂ register u32 r1 asm("r1") = arg1;
>>> +ÂÂÂ register u32 r2 asm("r2") = arg2;
>>> +
>>> ÂÂÂÂÂ asm volatile(
>>> ÂÂÂÂÂÂÂÂÂ ".arch_extensionÂÂÂ sec\n\t"
>>> -ÂÂÂÂÂÂÂ "stmfdÂÂÂ sp!, {r4 - r11, lr}\n\t"
>>> +ÂÂÂÂÂÂÂ "stmfdÂÂÂ sp!, {r4 - r11}\n\t"
>>> ÂÂÂÂÂÂÂÂÂ __asmeq("%0", "r0")
>>> ÂÂÂÂÂÂÂÂÂ __asmeq("%1", "r1")
>>> ÂÂÂÂÂÂÂÂÂ __asmeq("%2", "r2")
>>> ÂÂÂÂÂÂÂÂÂ "movÂÂÂ r3, #0\n\t"
>>> ÂÂÂÂÂÂÂÂÂ "movÂÂÂ r4, #0\n\t"
>>> ÂÂÂÂÂÂÂÂÂ "smcÂÂÂ #0\n\t"
>>> -ÂÂÂÂÂÂÂ "ldmfdÂÂÂ sp!, {r4 - r11, pc}"
>>> +ÂÂÂÂÂÂÂ "ldmfdÂÂÂ sp!, {r4 - r11}\n\t"
>>> ÂÂÂÂÂÂÂÂÂ :
>>> -ÂÂÂÂÂÂÂ : "r" (type), "r" (arg1), "r" (arg2)
>>> -ÂÂÂÂÂÂÂ : "memory");
>>> +ÂÂÂÂÂÂÂ : "r" (r0), "r" (r1), "r" (r2)
>>> +ÂÂÂÂÂÂÂ : "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.