how CPU hot-plug is suppose to work on Linux?

how CPU hot-plug is suppose to work on Linux?

[Gleb Natapov]

Hello,

We are trying to add CPU hot-plug/unplug capability to KVM. We want to
be able to initiate hot-plug/unplug from a host. Our current schema
works like this:

We have Processor object in DSDT for each potentially available CPU.
Each Processor object has _MAD, _STA, _EJ0. _MAD of present CPU returns
enabled LAPIC structure. _STA of present CPU return 0xf. _MADT of non
present CPU returns disabled LAPIC. _STA returns 0x0. _EJ0 does nothing.

When CPU is hot plugged:

 1. Bit is set in sts register of gpe
 2. acpi interrupt is sent
 3. Linux ACPI evaluates corespondent gpe's _L() method
 4. _L() method determines which CPU's status is changed
 5. For each CPU that changed status from not present to present
    call Notify(1) to corespondent Processor() object.

When CPU is hot unplugged:

 1. Bit is set in sts register of gpe
 2. acpi interrupt is sent
 3. Linux ACPI evaluates corespondent gpe's _L() method
 4. _L() method determines which CPU's status is changed
 5. For each CPU that changed status from present to non present
    call Notify(3) to corespondent Processor() object.

Now, CPU hot plug appears to be working. But CPU hot unplug does
nothing. I expect that Linux will offline CPU and eject it after
evaluating Notify(3) and seeing that _STA of ejected CPU returns
0x0 now.

Any ideas how it is suppose to work?

--
			Gleb.

Re: how CPU hot-plug is suppose to work on Linux?

[Thomas Renninger]

On Tuesday 21 September 2010 15:41:09 Gleb Natapov wrote:
> Hello,
> 
> We are trying to add CPU hot-plug/unplug capability to KVM. We want to
> be able to initiate hot-plug/unplug from a host. Our current schema
> works like this:
> 
> We have Processor object in DSDT for each potentially available CPU.
> Each Processor object has _MAD, _STA, _EJ0. _MAD of present CPU
> returns enabled LAPIC structure. _STA of present CPU return 0xf. _MADT
> of non present CPU returns disabled LAPIC. _STA returns 0x0. _EJ0 does
> nothing.
> 
> When CPU is hot plugged:
> 
>  1. Bit is set in sts register of gpe
>  2. acpi interrupt is sent
>  3. Linux ACPI evaluates corespondent gpe's _L() method
>  4. _L() method determines which CPU's status is changed
>  5. For each CPU that changed status from not present to present
>     call Notify(1) to corespondent Processor() object.
> 
> When CPU is hot unplugged:
> 
>  1. Bit is set in sts register of gpe
>  2. acpi interrupt is sent
>  3. Linux ACPI evaluates corespondent gpe's _L() method
>  4. _L() method determines which CPU's status is changed
>  5. For each CPU that changed status from present to non present
>     call Notify(3)
That does not work.

>     to corespondent Processor() object.
> 
> Now, CPU hot plug appears to be working. But CPU hot unplug does
> nothing. I expect that Linux will offline CPU and eject it after
> evaluating Notify(3) and seeing that _STA of ejected CPU returns
> 0x0 now.
> 
> Any ideas how it is suppose to work?
Put a container device, e.g. ACPI0004 above the CPU objects.
and call notify (on remove and add) on it instead of the CPU
object itself with e.g.:
#define ACPI_NOTIFY_BUS_CHECK           (u8) 0x00
#define ACPI_NOTIFY_DEVICE_CHECK        (u8) 0x01

acpi core should go through the devices and call add()/probe()
on devices which appear and remove() on devices which _STA
method does not return active anymore.

    Thomas