[Qemu-devel] Re: [Bochs-developers] [PATCH 1/2] create acpi cpu definitions

[Gleb Natapov]

On Sun, Apr 19, 2009 at 11:01:09AM +0930, Brendan Trotter wrote:
> Hi,
> 
> On Sun, Apr 19, 2009 at 7:20 AM, Sebastian Herbszt <address@hidden> wrote:
> > Glauber Costa wrote:
> >> This comes directly from kvm-userspace. It creates
> >> the necessary infrastructure for cpu hotplug, by
> >> creating _MAT and _STA entries in cpu devices,
> >> and by allowing notifications to the guest to happen
> >
> > Is there a cpu hotplug specification? I would like to read up
> > on the needed changes.
> 
> There isn't any CPU hotplug specification for 80x86.
> 
> The "Processor Local APIC" structure in ACPI's tables has an
> "enabled/disabled" flag (just like some other structures in ACPI
> tables). Hotplug CPUs that aren't present may not be listed at all (no
> "disabled" entry), and "Processor Local APIC" entries may be disabled
> for any number of other reasons (including a BIOS that uses fixed size
> tables, that supports more CPUs than the motherboard). The only thing
> an OS can assume about disabled "Processor Local APIC" entries is that
> the OS must not attempt to use the CPU.
> 
> For a comparison, the ACPI specification does include support for
> hotplug RAM. In this case (for ACPI 3.0) the "int 0x15, eax = 0xE820"
> BIOS function returns entries with an enabled/disabled flag where
> "disabled" entries must be ignored; and there's a completely separate
> structure (the "Memory Affinity Structure") which contains information
> about areas that are used for hot-plug RAM, which has it's one
> enabled/disabled flag *and* a separate hotpluggable/not hotpluggable
> flag. From this, it seems logical that if ACPI ever does support
> hot-plug CPUs, then they'll use a separate structure or a separate
> flag to indicate if a CPU is hot pluggable or not, and the existing
> "enabled/disabled" flag will retain it's current (use/don't use)
> meaning.
> 
> For some reason (unknown to me) some Linux developers made wild
> assumptions about disabled "Processor Local APIC" entries, and now
> they're inventing fictitious hardware to support their unfounded
> assumptions.
> 
> Please, correct me if I'm wrong...
> 
Windows 2008 supports CPU hot plug (but not unplug IIRC). How they do it
if there is not specification about how it should work on 80x86?

--
                        Gleb.