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

From: Benjamin Gray <[email-masked]>
Date: Thu, 30 Mar 2023 16:50:34 +1100

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 <[email-masked]>

---
New in v1

Preface with: I'm note sure on the best place to put the config.

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
+	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
+	  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);
 }
 
-- 
2.39.2