undefined instruction d5380001 (arm64 mrs emulation)

[Matwey V. Kornilov]

The patch helps to overcome the issue, Probably it should be applied
to all stable releases affected by this behaviour.
modprobe in initrd may load quite required things.


2017-10-02 18:56 GMT+03:00 Suzuki K Poulose <Suzuki.Poulose at arm.com>:
> On Mon, Oct 02, 2017 at 03:11:18PM +0100, James Morse wrote:
>> Hi Matwey,
>>
>> On 02/10/17 12:24, Dave Martin wrote:
>> > On Fri, Sep 29, 2017 at 10:23:54PM +0300, Matwey V. Kornilov wrote:
>> >> I am running 4.13.3 on rockchip 3328 platform(aarch64) with glibc 2.26
>> >> and see the following at booting:
>> >>
>> >> [   11.152061] modprobe[93]: undefined instruction: pc=0000ffff8ca48ff4
>> >> [   11.152707] Code: d503201f 8a180320 92750001 365ffc20 (d5380001)
>> >> [   11.154347] modprobe[94]: undefined instruction: pc=0000ffff94243ff4
>> >> [   11.154991] Code: d503201f 8a180320 92750001 365ffc20 (d5380001)
>> >> [   11.157070] modprobe[97]: undefined instruction: pc=0000ffff839a0ff4
>> >> [   11.157715] Code: d503201f 8a180320 92750001 365ffc20 (d5380001)
>> >> [   11.159265] modprobe[98]: undefined instruction: pc=0000ffffb0591ff4
>> >> [   11.159908] Code: d503201f 8a180320 92750001 365ffc20 (d5380001)
>> >>
>> >> As far as I understand d5380001 should be emulated in cpufeature.c but
>> >> it is not. What could be wrong here?
>> >
>> > The whole sequence is
>> >
>> >    0:   d503201f        nop
>> >    4:   8a180320        and     x0, x25, x24
>> >    8:   92750001        and     x1, x0, #0x800
>> >    c:   365ffc20        tbz     w0, #11, 0xffffffffffffff90
>> >   10:*  d5380001        mrs     x1, midr_el1            <-- trapping instruction
>>
>> This looks the same as:
>> https://bugzilla.redhat.com/show_bug.cgi?id=1496209
>>
>> [...]
>>
>> > What should happen here is that the do_undefinstr() in
>> > arch/arm64/kernel/traps.c should call registered undef hooks until it
>> > finds one that accepts the faulting instruction.
>> >
>> > So, either the cpufeatures undef hook is not getting called, or it is
>> > failing the instruction somewhere, possibly in
>> > cpufeatures.c:emulate_id_reg() or emulate_sys_reg().
>> >
>> >
>> > Can you add some trace to those functions to see what's happening?
>>
>> I couldn't reproduce this with linux-stable's v4.13.3 defconfig on Seattle or Juno.
>>
>> What distribution are you running? Could you also try [0] to see if this is
>> something specific to your version of modprobe?
>
>
> It is worth noting that we register the MRS instruction handler as late_init call.
> Now the question is how late that could be. Given that we are hitting it with
> modprobe, which could be used for requesting modules from initrd. Also which explains
> why it we can't reproduce it by simple testcases, after it was registered.
>
> Now the question is, how early do we want to push this. Since it doesn't depend really
> on any other subsystem, we could move it as early as "early". Or for keeping it in
> line with other "arch" specific init calls, we could simply make it arch_initcall.
>
> Matwey,
>
> Please could you check if the following patch fixes the issue for you:
>
> Cheers
> Suzuki
>
> ----8>----
>
> arm64: Enable MRS emulation early enough in the boot sequence
>
> Make sure the MRS emulation is enabled early enough that the
> early userspace applications (e.g, those run from initrd) could
> run without any trouble.
>
> Signed-off-by: Suzuki K Poulose <suzuki.poulose at arm.com>
>
> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index 9f9e0064c8c1..048f5469531f 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -1294,4 +1294,4 @@ static int __init enable_mrs_emulation(void)
>         return 0;
>  }
>
> -late_initcall(enable_mrs_emulation);
> +arch_initcall(enable_mrs_emulation);
> ---
>
>
>>
>>
>> Thanks,
>>
>> James
>>
>> [0] works for me:
>> ---------------------%<---------------------
>> #include <stdio.h>
>> #include <sys/auxv.h>
>>
>> #ifndef HWCAP_CPUID
>> #define HWCAP_CPUID (1 << 11)
>> #endif
>>
>> int main(int argc, char **argv)
>> {
>>         register unsigned int midr asm ("r1") = 0;
>>         unsigned long hwcaps = getauxval(AT_HWCAP);
>>
>>         if (!(hwcaps & HWCAP_CPUID)) {
>>                 fprintf(stderr, "mrs emulation not supported\n");
>>                 return 1;
>>         }
>>
>>         asm("mrs %0, midr_el1" : "=r"(midr));
>>
>>         fprintf(stderr, "mrs x1, midr_el1; x1=0x%x\n", midr);
>>
>>         return 0;
>> }
>> ---------------------%<---------------------
>>



-- 
With best regards,
Matwey V. Kornilov