[PATCH v2 06/12] powerpc/dexcr: Support custom default DEXCR value

[Russell Currey]

On Thu, 2023-03-30 at 16:50 +1100, Benjamin Gray wrote:
> Make the DEXCR value configurable at config time. Intentionally don't
> limit possible values to support future aspects without needing
> kernel
> updates.
> 
> The default config value enables hashst/hashchk in problem state.
> This should be safe, as generally software needs to request these
> instructions be included in the first place.
> 
> Signed-off-by: Benjamin Gray <bgray at linux.ibm.com>
> 

Reviewed-by: Russell Currey <ruscur at russell.cc>

> ---
> New in v1
> 
> Preface with: I'm note sure on the best place to put the config.

Me neither.

> 
> I also don't think there's any need to zero out unknown/unsupported
> bits. Reserved implies they are ignored by the hardware (from my
> understanding of the ISA). Current P10s boot with all bits set;
> lsdexcr
> (later patch) reports
> 
>    uDEXCR: ff000000 (SBHE, IBRTPD, SRAPD, NPHIE, PHIE, unknown)
> 
> when you try to read it back. Leaving them be also makes it easier to
> support newer aspects without a kernel update.
> 
> If arbitrary value support isn't important, it's probably a nicer
> interface to make each aspect an entry in a menu.
> 
> Future work may include dynamic DEXCR controls via prctl() and sysfs.
> The dynamic controls would be able to override this default DEXCR on
> a
> per-process basis. A stronger "PPC_ENFORCE_USER_ROP_PROCTETION"
> config
> may be required at such a time to prevent dynamically disabling the
> hash checks.
> ---
>  arch/powerpc/Kconfig                  | 14 ++++++++++++++
>  arch/powerpc/kernel/cpu_setup_power.c |  3 ++-
>  2 files changed, 16 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
> index 47017975fc2b..809ae576e19f 100644
> --- a/arch/powerpc/Kconfig
> +++ b/arch/powerpc/Kconfig
> @@ -1035,6 +1035,20 @@ config PPC_MEM_KEYS
>  
>           If unsure, say y.
>  
> +config PPC_DEXCR_DEFAULT

Calling it "default" could be slightly misleading since there's no
other way to modify the DEXCR right now.  It'll make more sense once
it's user configurable though.


> +       hex "Default DEXCR value"
> +       default 0x0000000004000000
> +       depends on PPC_BOOK3S_64
> +       help
> +         Power10 introduces the Dynamic Execution Control Register
> (DEXCR)
> +         to provide fine grained control over various speculation
> and
> +         security capabilities. This is used as the default DEXCR
> value.
> +
> +         It is a 64 bit value that splits into 32 bits for
> supervisor mode
> +         and 32 bits for problem state. The default config value
> enables
> +         the hashst/hashck instructions in userspace. See the ISA
> for

hashchk*

May also be useful to reference the ISA version here.

> +         specifics of what each bit controls.
> +
>  config PPC_SECURE_BOOT
>         prompt "Enable secure boot support"
>         bool
> diff --git a/arch/powerpc/kernel/cpu_setup_power.c
> b/arch/powerpc/kernel/cpu_setup_power.c
> index c00721801a1b..814c825a0661 100644
> --- a/arch/powerpc/kernel/cpu_setup_power.c
> +++ b/arch/powerpc/kernel/cpu_setup_power.c
> @@ -10,6 +10,7 @@
>  #include <asm/reg.h>
>  #include <asm/synch.h>
>  #include <linux/bitops.h>
> +#include <linux/kconfig.h>
>  #include <asm/cputable.h>
>  #include <asm/cpu_setup.h>
>  
> @@ -128,7 +129,7 @@ static void init_PMU_ISA31(void)
>  
>  static void init_DEXCR(void)
>  {
> -       mtspr(SPRN_DEXCR, 0);
> +       mtspr(SPRN_DEXCR, CONFIG_PPC_DEXCR_DEFAULT);
>         mtspr(SPRN_HASHKEYR, 0);
>  }
>