powerpc/dexcr: Track the DEXCR per-process

Add capability to make the DEXCR act as a per-process SPR.

We do not yet have an interface for changing the values per task. We
also expect the kernel to use a single DEXCR value across all tasks
while in privileged state, so there is no need to synchronize after
changing it (the userspace aspects will synchronize upon returning to
userspace).
Signed-off-by: Benjamin Gray <bgray@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240417112325.728010-3-bgray@linux.ibm.com

arch/powerpc/include/asm/processor.h

@@ -260,6 +260,7 @@ struct thread_struct {
 	unsigned long   sier2;
 	unsigned long   sier3;
 	unsigned long	hashkeyr;
+	unsigned long	dexcr;
 
 #endif
 };

arch/powerpc/kernel/process.c

@@ -1185,6 +1185,9 @@ static inline void save_sprs(struct thread_struct *t)
 
 	if (cpu_has_feature(CPU_FTR_DEXCR_NPHIE))
 		t->hashkeyr = mfspr(SPRN_HASHKEYR);
+
+	if (cpu_has_feature(CPU_FTR_ARCH_31))
+		t->dexcr = mfspr(SPRN_DEXCR);
 #endif
 }
 
@@ -1267,6 +1270,10 @@ static inline void restore_sprs(struct thread_struct *old_thread,
 	if (cpu_has_feature(CPU_FTR_DEXCR_NPHIE) &&
 	    old_thread->hashkeyr != new_thread->hashkeyr)
 		mtspr(SPRN_HASHKEYR, new_thread->hashkeyr);
+
+	if (cpu_has_feature(CPU_FTR_ARCH_31) &&
+	    old_thread->dexcr != new_thread->dexcr)
+		mtspr(SPRN_DEXCR, new_thread->dexcr);
 #endif
 
 }
@@ -1878,6 +1885,9 @@ int copy_thread(struct task_struct *p, const struct kernel_clone_args *args)
 #ifdef CONFIG_PPC_BOOK3S_64
 	if (cpu_has_feature(CPU_FTR_DEXCR_NPHIE))
 		p->thread.hashkeyr = current->thread.hashkeyr;
+
+	if (cpu_has_feature(CPU_FTR_ARCH_31))
+		p->thread.dexcr = mfspr(SPRN_DEXCR);
 #endif
 	return 0;
 }

arch/powerpc/kernel/ptrace/ptrace-view.c

@@ -469,12 +469,7 @@ static int dexcr_get(struct task_struct *target, const struct user_regset *regse
 	if (!cpu_has_feature(CPU_FTR_ARCH_31))
 		return -ENODEV;
 
-	/*
-	 * The DEXCR is currently static across all CPUs, so we don't
-	 * store the target's value anywhere, but the static value
-	 * will also be correct.
-	 */
-	membuf_store(&to, (u64)lower_32_bits(DEXCR_INIT));
+	membuf_store(&to, (u64)lower_32_bits(target->thread.dexcr));
 
 	/*
 	 * Technically the HDEXCR is per-cpu, but a hypervisor can't reasonably