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

From: Gleb Natapov
Subject: [Qemu-devel] Re: [Bochs-developers] [PATCH 1/2] create acpi cpu definitions
Date: Sun, 19 Apr 2009 14:12:49 +0300

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?


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