Date:   Tue, 17 Apr 2018 10:11:10 +0200
From:   Thierry Reding <treding@...dia.com>
To:     Stefan Agner <stefan@...er.ch>,
        Russell King <linux@...linux.org.uk>
CC:     Stephen Warren <swarren@...dotorg.org>, <linux@...linux.org.uk>,
        "Stephen Warren" <swarren@...dia.com>,
        Robin Murphy <robin.murphy@....com>,
        <ard.biesheuvel@...aro.org>, <arnd@...db.de>,
        <nicolas.pitre@...aro.org>, <marc.zyngier@....com>,
        <behanw@...verseincode.com>, <keescook@...omium.org>,
        <Bernhard.Rosenkranzer@...aro.org>, <mka@...omium.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>, Dmitry Osipenko <digetx@...il.com>
Subject: Re: [PATCH v2 3/6] ARM: trusted_foundations: do not use naked
 function

On Mon, Apr 16, 2018 at 08:21:09PM +0200, 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@...il.com>
> >>>>>> Cc: Stephen Warren <swarren@...dia.com>
> >>>>>> Cc: Thierry Reding <treding@...dia.com>
> >>>>>> Signed-off-by: Stefan Agner <stefan@...er.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 don't mind picking this up into the Tegra tree. Might be a good idea
to move this into drivers/firmware, though, since that's where all the
other firmware-related drivers reside.

Firmware code, such as the BPMP driver, usually goes through ARM-SoC
these days. I think this is in the same category.

Russell, any objections to me picking this patch up and moving it into
drivers/firmware?

Thanks,
Thierry

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