[PATCH v2 02/20] powerpc: Always restore FPU/VEC/VSX if hardware transactional memory in use

Cyril Bur cyrilbur at gmail.com
Fri Aug 12 09:28:01 AEST 2016

Comment from arch/powerpc/kernel/process.c:967:
 If userspace is inside a transaction (whether active or
 suspended) and FP/VMX/VSX instructions have ever been enabled
 inside that transaction, then we have to keep them enabled
 and keep the FP/VMX/VSX state loaded while ever the transaction
 continues.  The reason is that if we didn't, and subsequently
 got a FP/VMX/VSX unavailable interrupt inside a transaction,
 we don't know whether it's the same transaction, and thus we
 don't know which of the checkpointed state and the ransactional
 state to use.

restore_math() restore_fp() and restore_altivec() currently may not
restore the registers. It doesn't appear that this is more serious
than a performance penalty. If the math registers aren't restored the
userspace thread will still be run with the facility disabled.
Userspace will not be able to read invalid values. On the first access
it will take an facility unavailable exception and the kernel will
detected an active transaction, at which point it will abort the
transaction. There is the possibility for a pathological case
preventing any progress by transactions, however, transactions
are never guaranteed to make progress.

Fixes: 70fe3d9 ("powerpc: Restore FPU/VEC/VSX if previously used")
Signed-off-by: Cyril Bur <cyrilbur at gmail.com>
 arch/powerpc/kernel/process.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/arch/powerpc/kernel/process.c b/arch/powerpc/kernel/process.c
index 58ccf86..a5cdef9 100644
--- a/arch/powerpc/kernel/process.c
+++ b/arch/powerpc/kernel/process.c
@@ -208,7 +208,7 @@ void enable_kernel_fp(void)
 static int restore_fp(struct task_struct *tsk) {
-	if (tsk->thread.load_fp) {
+	if (tsk->thread.load_fp || MSR_TM_ACTIVE(tsk->thread.regs->msr)) {
 		return 1;
@@ -278,7 +278,8 @@ EXPORT_SYMBOL_GPL(flush_altivec_to_thread);
 static int restore_altivec(struct task_struct *tsk)
-	if (cpu_has_feature(CPU_FTR_ALTIVEC) && tsk->thread.load_vec) {
+	if (cpu_has_feature(CPU_FTR_ALTIVEC) &&
+		(tsk->thread.load_vec || MSR_TM_ACTIVE(tsk->thread.regs->msr))) {
 		tsk->thread.used_vr = 1;
@@ -464,7 +465,8 @@ void restore_math(struct pt_regs *regs)
 	unsigned long msr;
-	if (!current->thread.load_fp && !loadvec(current->thread))
+	if (!MSR_TM_ACTIVE(regs->msr) &&
+		!current->thread.load_fp && !loadvec(current->thread))
 	msr = regs->msr;

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