Re: [Qemu-devel] [PATCH 11/19] target-i386: introduce apic-id property

[Igor Mammedov <imammedo@redhat.com>]

On Mon, 15 Apr 2013 11:49:40 -0300
Eduardo Habkost <ehabkost@redhat.com> wrote:

> On Mon, Apr 15, 2013 at 04:34:41PM +0200, Igor Mammedov wrote:
> > On Mon, 15 Apr 2013 10:45:24 -0300
> > Eduardo Habkost <ehabkost@redhat.com> wrote:
> > 
> > > On Mon, Apr 15, 2013 at 03:37:00PM +0200, Igor Mammedov wrote:
> > > [...]
> > > > > > > ID to be directly specified in the device_add/-device options.
> > > > > > > That's how real CPUs work: as the CPU manufacturer doesn't know
> > > > > > > what will be the package ID of the CPU, the APIC IDs are not
> > > > > > > hardcoded in the CPU; they are calculated based on the CPU topology
> > > > > > > and some socket identifier signal coming from the board.
> > > > > > that is why apic_id has been made a property, to be set from outside.
> > > > > 
> > > > > True. I believe the conflict here is: we want other objects to set the
> > > > > APIC ID (be it the board, or socket/core objects), but at the same time
> > > > > it would be interesting to not expose the APIC ID outside QEMU. Being
> > > > > too flexible regarding the APIC ID is more likely to cause problems
> > > > > later.
> > > > > 
> > > > > That said, I don't mind having a "apic-id" property because it is easier
> > > > > to simply expose it directly. But do you agree that: 1) we don't really
> > > > > need to expose it to be set from outside QEMU; 2) we shouldn't require
> > > > > it to be set from outside QEMU; 3) we should recommend users to not try
> > > > > to fiddle it with?
> > > > Due to nature of per thread CPU hotplug, management will have to specify
> > > > some kind of ID to specify which CPU is being plugged. Management really
> > > > doesn't/shouldn't care what this ID is.
> > > 
> > > As long as management really doesn't/shouldn't care what the ID is,
> > > exposing the APIC ID in the form of an opaque CPU identifier wouldn't be
> > > a problem to me. I just wanted to make clarify if we agree that messing
> > > with the APIC ID directly won't be recommended and that the "apic-id"
> > > property will be for QEMU internal use only.
> > On contrary, it's useful external feature, x86 guests see only APIC ID, since
> > it's the only ID they [should] know about. So guest aware mgmt could
> > definitely use apic_id propery to correlate CPU in guest with QEMU view of
> > them.
> 
> You're right, _reading_ the APIC ID is very useful. I am worried about
> _setting_ it from external code.

currently it's not possible since cpu-add doesn't allow to set any properties.

We will need setting it for device_add though.
By then, I guess some way to check that it's valid would be enough, otherwise
hot-plugged CPU will be out of scope of MADT and guest would ignore it or
through an error.

> 
> -- 
> Eduardo
> 


-- 
Regards,
  Igor