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Subject[PATCH v2 14/18] powerpc: Define set_thread_used_vas()
Date
A CP_ABORT instruction is required in processes that have mapped a VAS
"paste address" with the intention of using COPY/PASTE instructions.
But since CP_ABORT is expensive, we want to restrict it to only processes
that use/intend to use COPY/PASTE.

Define an interface, set_thread_used_vas(), that VAS can use to indicate
that the current process opened a send window. During context switch,
issue CP_ABORT only for processes that have the flag set.

Thanks for input from Nick Piggin, Michael Ellerman.

Signed-off-by: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>
---
arch/powerpc/include/asm/processor.h | 2 ++
arch/powerpc/include/asm/switch_to.h | 2 ++
arch/powerpc/kernel/process.c | 32 ++++++++++++++++++++++----------
3 files changed, 26 insertions(+), 10 deletions(-)

diff --git a/arch/powerpc/include/asm/processor.h b/arch/powerpc/include/asm/processor.h
index 58cc212..bdab3b74 100644
--- a/arch/powerpc/include/asm/processor.h
+++ b/arch/powerpc/include/asm/processor.h
@@ -341,7 +341,9 @@ struct thread_struct {
unsigned long sier;
unsigned long mmcr2;
unsigned mmcr0;
+
unsigned used_ebb;
+ unsigned int used_vas;
#endif
};

diff --git a/arch/powerpc/include/asm/switch_to.h b/arch/powerpc/include/asm/switch_to.h
index f5da32f..aeb305b 100644
--- a/arch/powerpc/include/asm/switch_to.h
+++ b/arch/powerpc/include/asm/switch_to.h
@@ -91,6 +91,8 @@ static inline void clear_task_ebb(struct task_struct *t)
#endif
}

+extern int set_thread_used_vas(void);
+
extern int set_thread_tidr(struct task_struct *t);
extern void clear_thread_tidr(struct task_struct *t);

diff --git a/arch/powerpc/kernel/process.c b/arch/powerpc/kernel/process.c
index d861fcd..cb5f108 100644
--- a/arch/powerpc/kernel/process.c
+++ b/arch/powerpc/kernel/process.c
@@ -1234,17 +1234,17 @@ struct task_struct *__switch_to(struct task_struct *prev,
* The copy-paste buffer can only store into foreign real
* addresses, so unprivileged processes can not see the
* data or use it in any way unless they have foreign real
- * mappings. We don't have a VAS driver that allocates those
- * yet, so no cpabort is required.
+ * mappings. If the new process has the foreign real address
+ * mappings, we must issue a cp_abort to clear any state and
+ * prevent a covert channel being setup.
+ *
+ * DD1 allows paste into normal system memory so we do an
+ * unpaired copy, rather than cp_abort, to clear the buffer,
+ * since cp_abort is quite expensive.
*/
- if (cpu_has_feature(CPU_FTR_POWER9_DD1)) {
- /*
- * DD1 allows paste into normal system memory, so we
- * do an unpaired copy here to clear the buffer and
- * prevent a covert channel being set up.
- *
- * cpabort is not used because it is quite expensive.
- */
+ if (new_thread->used_vas) {
+ asm volatile(PPC_CP_ABORT);
+ } else if (cpu_has_feature(CPU_FTR_POWER9_DD1)) {
asm volatile(PPC_COPY(%0, %1)
: : "r"(dummy_copy_buffer), "r"(0));
}
@@ -1445,6 +1445,18 @@ void flush_thread(void)
#endif /* CONFIG_HAVE_HW_BREAKPOINT */
}

+int set_thread_used_vas(void)
+{
+#ifdef CONFIG_PPC_BOOK3S_64
+ if (!cpu_has_feature(CPU_FTR_ARCH_300))
+ return -EINVAL;
+
+ current->thread.used_vas = 1;
+
+#endif /* CONFIG_PPC_BOOK3S_64 */
+ return 0;
+}
+
#ifdef CONFIG_PPC64
static DEFINE_SPINLOCK(vas_thread_id_lock);
static DEFINE_IDA(vas_thread_ida);
--
2.7.4
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