[Qemu-devel] Re: [PATCH] return default values for apic probe functions.

[Avi Kivity <avi@redhat.com>]

Glauber Costa wrote:
> As KVM cpus runs on threads, it is possible that
> we call kvm_load_registers() from a cpu thread, while the
> apic has not yet fully initialized. kvm_load_registers() is called
> from ap_main_loop.
>
> This is not a problem when we're starting the whole machine together,
> but is a problem for hotplug, since we don't have the protection
> of the locks that protect machine initialization. Currently, some executions
> of cpu hotplug on rainy sundays fail with a segfault.
>
> Moving apic initialization to before kvm_init_vpcu proved fruitful,
> as there are some dependencies involved. (kvm irqchip would fail to
> initialize).
>   

I presume you mean unfruitful (or perhaps a nasty kind of fruit).

> This patch provides default values to be used for tpr and apic_base,
> that will be returned when the apic is not yet properly initialized.
> It is aimed at kvm, where the problem exists, but it could equally be
> used for qemu too, if there is agreement.
>   

Seems like a hack... can you try not to make the vcpu visible until it 
is completely initialized?

(and what is the problem exactly - someone accessing the registers from 
a different thread? that shouldn't happen)

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.