Re: [PATCH v6 0/9] powerpc: Switch to CONFIG_THREAD_INFO_IN_TASK

On Mon, 08 Oct 2018 20:59:56 +1100
Benjamin Herrenschmidt [off-list ref] wrote:

On Mon, 2018-10-08 at 09:16 +0000, Christophe Leroy wrote:

quoted

The purpose of this serie is to activate CONFIG_THREAD_INFO_IN_TASK which
moves the thread_info into task_struct.  

We need to make sure we don't have code that assumes that we don't take
faults on TI access.

On ppc64, the stack SLB entries are bolted, which means the TI is too.

We might have code that assumes that we don't get SLB faults when
accessing TI. If not, we're fine but that needs a close look.

Oh, we do. I think the entry side might be okay, but on exit we have
at least one (in syscall and interrupt exit both):

        /*
         * Disable interrupts so current_thread_info()->flags can't change,
         * and so that we don't get interrupted after loading SRR0/1.
         */
#ifdef CONFIG_PPC_BOOK3E
        wrteei  0
#else
        /*
         * For performance reasons we clear RI the same time that we
         * clear EE. We only need to clear RI just before we restore r13
         * below, but batching it with EE saves us one expensive mtmsrd call.
         * We have to be careful to restore RI if we branch anywhere from
         * here (eg syscall_exit_work).
         */
        li      r11,0
        mtmsrd  r11,1
#endif /* CONFIG_PPC_BOOK3E */

        ld      r9,TI_FLAGS(r12)

So taking an SLB there will cause an unrecoverable.

I think we can probably get rid of that optimization for now. I've found
for non-trivial syscalls it's often a loss if FP was used. I have a
couple of different options I'm working on to get rid of the mtmsrd
entirely we can go with instead (but I don't think those have to come
before Christophe's patch).

Thanks,
Nick