Re: [PATCH 2/2] ARM: soft-reboot into same mode that we entered the kernel

Marc Zyngier
2016-12-14T13:41:40Z

On 14/12/16 12:05, Russell King - ARM Linux wrote:
> On Wed, Dec 14, 2016 at 11:56:49AM +0000, Mark Rutland wrote:
>> On Wed, Dec 14, 2016 at 10:46:35AM +0000, Russell King wrote:
>>> When we soft-reboot (eg, kexec) from one kernel into the next, we need
>>> to ensure that we enter the new kernel in the same processor mode as
>>> when we were entered, so that (eg) the new kernel can install its own
>>> hypervisor - the old kernel's hypervisor will have been overwritten.
>>>
>>> In order to do this, we need to pass a flag to cpu_reset() so it knows
>>> what to do, and we need to modify the kernel's own hypervisor stub to
>>> allow it to handle a soft-reboot.
>>>
>>> As we are always guaranteed to install our own hypervisor if we're
>>> entered in HYP32 mode, and KVM will have moved itself out of the way
>>> on kexec/normal reboot, we can assume that our hypervisor is in place
>>> when we want to kexec, so changing our hypervisor API should not be a
>>> problem.
>>
>> Just to check, does that also hold true for kdump?
>>
>> I haven't gone digging yet, but it looks like KVM might still be
>> installed, rather than the hyp stub, and we might need some logic to
>> ensure that it's torn down...
>
> The same problem will be true of ARM64 - I don't see any solution to
> that in the present state.
>
>>> @@ -51,7 +52,9 @@ static void __soft_restart(void *addr)
>>>
>>> /* Switch to the identity mapping. */
>>> phys_reset = (phys_reset_t)virt_to_idmap(cpu_reset);
>>> - phys_reset((unsigned long)addr);
>>> +
>>> + /* original stub should be restored by kvm */
>>> + phys_reset((unsigned long)addr, is_hyp_mode_available());
>>
>> ... otherwise here we'd call into the KVM hyp code in a potentially
>> confusing manner.
>>
>> Otherwise, this looks fine to me.
>
> The only thing that I can think which would resolve that would be to
> lay down a standard API for the hyp code, so things like reboot into
> hyp mode can work irrespective of the hyp stub in place.

That looks like a sensible thing to do. I'll look into it.

Thanks,

M.
--
Jazz is not dead. It just smells funny...