Re: [PATCH] powerpc/tm: Set ckpt_regs.msr before using it.
From: Cyril Bur <hidden>
Date: 2017-10-24 22:39:15
On Tue, 2017-10-24 at 15:13 -0200, Breno Leitao wrote:
From: Breno Leitao <redacted>
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.I believe this analysis is correct, I have come to the same conclusion in the past. I've also done a bunch of testing with variants of this patch and haven't seen a difference, however, I do believe the code is more correct with this patch. Signed-off-by: Cyril Bur <redacted> Having said all that, nothing rules out that our tests simply aren't good enough ;)
quoted hunk ↗ jump to hunk
The solution I am proposing is updating ckpt_regs.msr before using it. Signed-off-by: Breno Leitao <leitao@debian.org> Signed-off-by: Gustavo Romero <redacted> CC: Cyril Bur <redacted> CC: Michael Neuling <redacted> --- 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); }