Re: [PATCH RFC 3/3] Break dependency between vcpu index in vcpus array and vcpu's apic id.

[Marcelo Tosatti <mtosatti@redhat.com>]

On Fri, May 22, 2009 at 02:41:20PM +0300, Gleb Natapov wrote:
> On Thu, May 21, 2009 at 05:29:57PM -0300, Marcelo Tosatti wrote:
> > On Thu, May 21, 2009 at 11:06:42PM +0300, Gleb Natapov wrote:
> > > > Why don't make a convention that vcpu_id 0 is the BSP, by default,
> > > > instead of the first vcpu created? This way if userspace creates vcpu 3
> > > > first, there are no problems.
> > > > 
> > > Because userspace no longer control vcpu_id allocation. I also want to
> > > get rid of "first vcpu is bsp" restriction, but this fill require new
> > > ioctl I guess.
> > 
> > Or hardcode "apic_id == 0 is BSP" for now, and later add an ioctl to set
> > the BSP?
> > 
> We should not assume anything about apic_ids. For instance if the number
> of ioapics+cpu cores are greater than 15 it is recommended to assign ids
> 0-(N-1) to ioapic (N is the number of ioapics). This is because ioapic
> id is only 4 bits long, but it occupies the same id space as apic ids.

If keep the behaviour like in the last patch submitted, and introduce a
new ioctl to set the BSP, you get backward compatibility for free.