Re: [RFC v6 20/62] powerpc: store and restore the pkey state across context switches

On Thu, Jul 27, 2017 at 02:32:59PM -0300, Thiago Jung Bauermann wrote:

Ram Pai [off-list ref] writes:

quoted

Store and restore the AMR, IAMR and UMOR register state of the task
before scheduling out and after scheduling in, respectively.

Signed-off-by: Ram Pai <redacted>

s/UMOR/UAMOR/

quoted

diff --git a/arch/powerpc/kernel/process.c b/arch/powerpc/kernel/process.c
index 2ad725e..9429361 100644
--- a/arch/powerpc/kernel/process.c
+++ b/arch/powerpc/kernel/process.c

@@ -1096,6 +1096,11 @@ static inline void save_sprs(struct thread_struct *t)
 		t->tar = mfspr(SPRN_TAR);
 	}
 #endif
+#ifdef CONFIG_PPC64_MEMORY_PROTECTION_KEYS
+	t->amr = mfspr(SPRN_AMR);
+	t->iamr = mfspr(SPRN_IAMR);
+	t->uamor = mfspr(SPRN_UAMOR);
+#endif
 }

 static inline void restore_sprs(struct thread_struct *old_thread,

@@ -1131,6 +1136,14 @@ static inline void restore_sprs(struct thread_struct *old_thread,
 			mtspr(SPRN_TAR, new_thread->tar);
 	}
 #endif
+#ifdef CONFIG_PPC64_MEMORY_PROTECTION_KEYS
+	if (old_thread->amr != new_thread->amr)
+		mtspr(SPRN_AMR, new_thread->amr);
+	if (old_thread->iamr != new_thread->iamr)
+		mtspr(SPRN_IAMR, new_thread->iamr);
+	if (old_thread->uamor != new_thread->uamor)
+		mtspr(SPRN_UAMOR, new_thread->uamor);
+#endif
 }

Shouldn't the saving and restoring of the SPRs be guarded by a check for
whether memory protection keys are enabled? What happens when trying to
access these registers on a CPU which doesn't have them?

Good point. need to guard it.  However; i think, these registers have been
available since power6.

RP