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 <bg...@linux.ibm.com>
>
Reviewed-by: Russell Currey <rus...@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);
> }
>
