Re: [Qemu-devel] [PATCH] spapr_cpu_core: instantiate CPUs separately

[Greg Kurz <groug@kaod.org>]

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On Tue, 17 Oct 2017 17:16:09 +1100
David Gibson <david@gibson.dropbear.id.au> wrote:

> On Mon, Oct 16, 2017 at 10:26:38AM +0200, Igor Mammedov wrote:
> > On Sat, 14 Oct 2017 20:33:37 +1100
> > David Gibson <david@gibson.dropbear.id.au> wrote:
> >   
> > > On Fri, Oct 13, 2017 at 01:31:44PM +0200, Greg Kurz wrote:  
> > > > The current code assumes that only the CPU core object holds a
> > > > reference on each individual CPU object, and happily frees their
> > > > allocated memory when the core is unrealized. This is dangerous
> > > > as some other code can legitimely keep a pointer to a CPU if it
> > > > calls object_ref(), but it would end up with a dangling pointer.
> > > > 
> > > > Let's allocate all CPUs with object_new() and let QOM frees them
> > > > when their reference count reaches zero. This greatly simplify the
> > > > code as we don't have to fiddle with the instance size anymore.
> > > > 
> > > > Signed-off-by: Greg Kurz <groug@kaod.org>    
> > > 
> > > So, I'm pretty sure my first drafts of the core stuff did things this
> > > waym and it got nacked, for QOM lifetime reasons that I never really
> > > understood.  
> > From what I remember, Andreas would like to see composite CPU object
> > allocated in one go and then its children initialized with object_initialize()
> > so that no more allocation were needed.  
> 
> Ah, ok.
> 
> > That potentially would benefit hotplug, since we could gracefully
> > fail object creation early if there is not enough memory.  
> 
> Yeah, it sounds nice, but I don't see how we can do it.  In order to
> do that the core object has to have enough space for all the threads,
> which means we need both the size of each thread object and the number
> of them.  The size we have (and will be easier to handle after Igor's
> cleanups).  The number, we don't.
> 
> > But the way it's implemented currently doesn't really match that initial
> > goal as array for threads is dynamically allocated later
> > and then we need to dance around it with pointer arithmetic.
> > 
> > BTW: almost any allocation failure in qemu currently
> > is fatal so whether we fail on array alloc or on individual
> > object_new() won't make any difference.
> > 
> > I'd rather see this clean up merged as it simplifies code
> > in these case.  
> 
> Ok, works for me.
> 

Since we're in soft freeze already, I guess this won't go to 2.11. Maybe it's
time to create ppc-for-2.12 and apply it there ?

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