[PATCH] powerpc/tm: Set ckpt_regs.msr before using it.

[Breno Leitao]

From: Breno Leitao <breno.leitao at debian.org>

On commit commit f48e91e87e67 ("powerpc/tm: Fix FP and VMX register
corruption"), we check ckpt_regs.msr to see if a feature (as VEC, VSX
and FP) is disabled (thus the hot registers might be bogus during the
reclaim), and then copy the previously saved thread registers, with the
non-bogus values, into the checkpoint area for a later trecheckpoint.
This mechanism is used to recheckpoints the proper register values when
a transaction started using the bogus registers, and these values were
sent to the memory checkpoint area.

I see a problem on this code that ckpt_regs.msr is not properly set when
using it, as for example, when there is a vsx_unavailable_tm() in a code
like the following, the ckpt_regs.msg[FP] is 0;

 1: sleep_until_{fp,vec,vsx} = 0
 2: fadd
 3: tbegin.
 4: beq
 5: xxmrghd
 6: tend.

In this case, line 5 will raise an vsx_unavailable_tm() exception, and
the ckpt_regs.msr[FP] will be zero before memcpy() block, executing the
memcpy even with the the FP registers hot. That is not correct because
we executed a float point instruction on line 2, and MSR[FP] was set to
1.

Fortunately this does not cause a big problem as I can see, other than
this extra memcpy() because treclaim() will later overwrite this wrong
copied value, since it relies on the correct MSR value, which was
updated by giveup_all->check_if_tm_restore_required. There might be a
problem when laziness is being turned on, but I was not able to
reproduce it.

The solution I am proposing is updating ckpt_regs.msr before using it.

Signed-off-by: Breno Leitao <leitao at debian.org>
Signed-off-by: Gustavo Romero <gusbromero at gmail.com>
CC: Cyril Bur <cyrilbur at gmail.com>
CC: Michael Neuling <mikey at neuling.org>
---
 arch/powerpc/kernel/process.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/arch/powerpc/kernel/process.c b/arch/powerpc/kernel/process.c
index c051dc2b42ad..773e9c5594e7 100644
--- a/arch/powerpc/kernel/process.c
+++ b/arch/powerpc/kernel/process.c
@@ -860,6 +860,9 @@ static void tm_reclaim_thread(struct thread_struct *thr,
 	if (!MSR_TM_SUSPENDED(mfmsr()))
 		return;
 
+	/* Give up all the registers and set ckpt_regs.msr */
+	giveup_all(container_of(thr, struct task_struct, thread));
+
 	/*
 	 * If we are in a transaction and FP is off then we can't have
 	 * used FP inside that transaction. Hence the checkpointed
@@ -879,8 +882,6 @@ static void tm_reclaim_thread(struct thread_struct *thr,
 		memcpy(&thr->ckvr_state, &thr->vr_state,
 		       sizeof(struct thread_vr_state));
 
-	giveup_all(container_of(thr, struct task_struct, thread));
-
 	tm_reclaim(thr, thr->ckpt_regs.msr, cause);
 }
 
-- 
2.14.2