[PATCH] Arm64: convert soft_restart() to assembly code

[Christoffer Dall]

On Mon, Aug 18, 2014 at 6:02 PM, Mark Rutland <mark.rutland at arm.com> wrote:
> Hi Geoff,
>
> On Fri, Aug 15, 2014 at 07:53:19PM +0100, Geoff Levand wrote:
>> Hi Mark,
>>
>> On Fri, 2014-08-15 at 19:21 +0100, Mark Rutland wrote:
>> > On Fri, Aug 15, 2014 at 06:20:21PM +0100, Geoff Levand wrote:
>> > > For the cpu-ops shutdown I'm working on I need a call to move the
>> > > secondary processors to an identity mapped spin loop after the identity
>> > > map is enabled.  I want to do this in C code, so it needs to happen
>> > > after the identity map is enabled, and before the dcache is disabled.
>> > >
>> > > I think to do this we can keep the existing soft_restart(addr) routine
>> > > with something like this:
>> > >
>> > > void soft_restart(unsigned long addr)
>> > > {
>> > >   setup_mm_for_reboot();
>> > >
>> > > #if defined(CONFIG_SMP)
>> > >   smp_secondary_shutdown();
>> > > #endif
>> > >
>> > >   cpu_soft_restart(addr);
>> > >
>> > >   /* Should never get here */
>> > >   BUG();
>> > > }
>> > >
>> >
>> > I don't follow why you need a hook in the middle of soft_restart. That
>> > sounds like a layering violation to me.
>> >
>> > I assume this is for implementing the spin-table cpu-return-addr idea?
>>
>> Yes.
>>
>> > If so, what's wrong with something like:
>>
>> > void spin_table_cpu_die(unsigned int cpu)
>> > {
>> >     unsigned long release_addr = per_cpu(return_addr, cpu);
>> >
>> >     /*
>> >      * We should have a local_disable(DBG|ASYNC|FIQ|IRQ) function or
>> >      * something similar as these are all context synchronising and
>> >      * therefore expensive.
>> >      */
>> >     local_dbg_disable();
>> >     local_async_disable();
>> >     local_fiq_disable();
>> >     arch_local_irq_disable();
>> >
>> >     soft_restart(release_addr);
>> > }
>>
>> OK, this is a much simpler way than what I was thinking, which
>> was to have the secondaries spin in the kernel until the main
>> cpu shutdown.  I'll switch over to this, thanks.
>
> I just realised that this is still missing the jump to EL2 that I
> mentioned a while back.
>
> I think what we need to do is:
>
> * Have KVM (if present) tears itself down prior to cpu_die, restoring
>   the __hyp_stub_vectors in VBAR_EL2 and disabling the MMU, and caches.
>
> * Add a mechanism to __hyp_stub_vectors to allow a hypercall to
>   call a function at EL2. We should be able to replace the current
>   hyp_stub el1_sync handler with that, and rework KVM to call a function
>   at EL2 to setup VBAR_EL2 appropriately at init time.
>
Why do you need to change the current mechanism?  Is this due to the
CPU being in a different state when restarted with the MMU enabled in
EL2 or something like that?

-Christoffer