Re: [PATCH 1/2] KVM: X86: Fix setup the virt_spin_lock_key before static key get initialized

[Davidlohr Bueso <dave@stgolabs.net>]

On Sat, 24 Mar 2018, Wanpeng Li wrote:

>Note: Peterz pointed out in the IRC we have to audit all the architectures that
>implement smp_prepare_boot_cpu() to see what they depend on if we want to move
>jump_label_init() before smp_prepare_boot_cpu(). So what this patch does is
>similar to the issue which handled in xen ca5d376e.

After some auditing, the jump_label_init() being moved before smp_prepare_boot_cpu()
seems fine, however, bulky mechanisms to update text segments conflict with early smp
bootup stages, such as this patch. So, while the disabling virt_spin_lock_key would
be done correctly _after_ jump_label_init(), it is still fragile in that we want to
be using lightweight patching such as jump_label_transform_static() -- which doesn't
take the text_mutex (blocking is out of the question), for example.

For pretty much all archs this means using the transform_static() version. For example
x86, this means using text_poke_early(). -- also ouchy on the !PageReserved(pages[0])
warning for text_poke().

I'm not sure yet of the best way to teach jump_label_transform() to behave like
jump_label_transform_static() under pre-smp bootup, such as when disabling hypervisor
pvspinlocks.

The s390 implementation seems safe as is given that stop_machine is now safe for early
pre-smp boot.

Thanks,
Davidlohr