Re: [RFC v2 1/6] x86: introduce kernel restartable sequence

[Nadav Amit <namit@vmware.com>]

> On Dec 31, 2018, at 12:08 PM, Andy Lutomirski <luto@kernel.org> wrote:
> 
> On Sun, Dec 30, 2018 at 11:20 PM Nadav Amit <namit@vmware.com> wrote:
>> It is sometimes beneficial to have a restartable sequence - very few
>> instructions which if they are preempted jump to a predefined point.
>> 
>> To provide such functionality on x86-64, we use an empty REX-prefix
>> (opcode 0x40) as an indication for instruction in such a sequence. Before
>> calling the schedule IRQ routine, if the "magic" prefix is found, we
>> call a routine to adjust the instruction pointer.  It is expected that
>> this opcode is not in common use.
>> 
>> The following patch will make use of this function. Since there are no
>> other users (yet?), the patch does not bother to create a general
>> infrastructure and API that others can use for such sequences. Yet, it
>> should not be hard to make such extension later.
> 
> The following patch does not use it.  Can you update this?

I will. Sorry for not updating the commit-log. The GCC plugin, and various
requests (that I am not sure I fully agree with) really caused me to spit
some blood.

>> +asmlinkage __visible void restart_kernel_rseq(struct pt_regs *regs)
>> +{
>> +       if (user_mode(regs) || *(u8 *)regs->ip != KERNEL_RESTARTABLE_PREFIX)
>> +               return;
> 
> else?
> 
> I suspect something is missing here.  Or I'm very confused.

Indeed, the code should call optpoline_restart_rseq() (or some other name,
once I fix the naming). I will fix it.