[PATCH] powerpc/64s: Fix NULL AT_BASE_PLATFORM when using DT CPU features

[Li Yi (Adam)]

> Date: Wed, 14 Mar 2018 10:14:11 +1100
> From: Michael Ellerman <mpe at ellerman.id.au>
> To: linuxppc-dev at ozlabs.org
> Cc: sukadev at linux.vnet.ibm.com, npiggin at gmail.com
> Subject: [PATCH] powerpc/64s: Fix NULL AT_BASE_PLATFORM when using DT
>         CPU features
> Message-ID: <20180313231411.9537-1-mpe at ellerman.id.au>
>
> When running virtualised the powerpc kernel is able to run the system
> in "compat mode" - which means the kernel and hardware are pretending
> to userspace that the CPU is an older version than it actually is.
>
> AT_BASE_PLATFORM is an AUXV entry that we export to userspace for use
> when we're running in that mode, which tells userspace the "platform"
> string for the real CPU version, as opposed to the faked version.
>
> Although we don't support compat mode when using DT CPU features, and
> arguably don't need to set AT_BASE_PLATFORM, the existing cputable
> based code always sets it even when we're running bare metal. That
> means the lack of AT_BASE_PLATFORM is a user-visible artifact of the
> fact that the kernel is using DT CPU features, which we don't want.
>
> So set it in the DT CPU features code also.
>
> This results in eg:
>   $ LD_SHOW_AUXV=1 /bin/true | grep "AT_.*PLATFORM"
>   AT_PLATFORM:     power9
>   AT_BASE_PLATFORM:power9
>

Is this issue related with DD2.2 CPU?
I tested on a Boston system with DD2.1 CPU, with kernel 4.14.23,
AT_BASE_PLATFORM value is correct:

[root at boston-sh-04 ~]# LD_SHOW_AUXV=1 /bin/true | grep "AT_.*PLATFORM"
AT_PLATFORM:     power9
AT_BASE_PLATFORM:power9
[root at boston-sh-04 ~]# uname -a
Linux boston-sh-04 4.14.23 #1 SMP Sat Mar 3 12:14:56 CST 2018 ppc64le
ppc64le ppc64le GNU/Linux

Thanks,
-Yi

> Signed-off-by: Michael Ellerman <mpe at ellerman.id.au>
> ---
>  arch/powerpc/kernel/dt_cpu_ftrs.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/arch/powerpc/kernel/dt_cpu_ftrs.c b/arch/powerpc/kernel/dt_cpu_ftrs.c
> index 945e2c29ad2d..0bcfb0f256e1 100644
> --- a/arch/powerpc/kernel/dt_cpu_ftrs.c
> +++ b/arch/powerpc/kernel/dt_cpu_ftrs.c
> @@ -720,6 +720,9 @@ static void __init cpufeatures_setup_finished(void)
>                 cur_cpu_spec->cpu_features |= CPU_FTR_HVMODE;
>         }
>
> +       /* Make sure powerpc_base_platform is non-NULL */
> +       powerpc_base_platform = cur_cpu_spec->platform;
> +
>         system_registers.lpcr = mfspr(SPRN_LPCR);
>         system_registers.hfscr = mfspr(SPRN_HFSCR);
>         system_registers.fscr = mfspr(SPRN_FSCR);
> --
> 2.14.1