Re: [PATCH] x86/acpi: keep x86_cpu_to_acpiid mapping valid on cpu hotplug

[Vitaly Kuznetsov <vkuznets@redhat.com>]

"Rafael J. Wysocki" <rafael@kernel.org> writes:

> On Mon, Feb 6, 2017 at 6:01 PM, Vitaly Kuznetsov <vkuznets@redhat.com> wrote:
>> We may or may not have all possible CPUs in MADT on boot but in any case
>> we're overwriting x86_cpu_to_acpiid mapping with U32_MAX when
>> acpi_register_lapic() is called again on the CPU hotplug path:
>> acpi_processor_hotadd_init() -> acpi_map_cpu() -> acpi_register_lapic().
>>
>> As we have the required acpi_id information in acpi_processor_hotadd_init()
>> propagate it to acpi_map_cpu() to always keep x86_cpu_to_acpiid mapping
>> valid.
>>
>> Reported-by: Andrew Jones <drjones@redhat.com>
>> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
>
> Is the bug report available and if so, do you have a pointer to it?
>

There was no 'real' bug, the issue was found by code inspection.
Potentially, the following scenario is currently broken:
- boot Xen HVM guest and do kdump on a secondary CPU
- with kdump kernel running try to hot-plug additional CPUs.
we'll end up with U32_MAX in x86_cpu_to_acpiid for hot-plugged CPUs so
we'll be assuming direct Linux<->Xen mapping which won't be the case and
all hypercalls referring to these CPUs will end up affecting some other
CPUs.

-- 
  Vitaly