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Re: [Qemu-ppc] [PATCH 17/25] spapr: add a sPAPRXive object to the machin

From: David Gibson
Subject: Re: [Qemu-ppc] [PATCH 17/25] spapr: add a sPAPRXive object to the machine
Date: Fri, 1 Dec 2017 15:14:23 +1100
User-agent: Mutt/1.9.1 (2017-09-22)

On Thu, Nov 30, 2017 at 03:15:09PM +0000, Cédric Le Goater wrote:
> On 11/30/2017 05:55 AM, David Gibson wrote:
> > On Thu, Nov 23, 2017 at 02:29:47PM +0100, Cédric Le Goater wrote:
> >> The XIVE object is designed to be always available, so it is created
> >> unconditionally on newer machines.
> > 
> > There doesn't actually seem to be anything dependent on machine
> > version here.
> No. I thought that was too early in the patchset. This is handled 
> in the last patch with a 'xive_exploitation' bool which is set to 
> false on older machines. 
> But, nevertheless, the XIVE objects are always created even if not
> used. Something to discuss.

That'll definitely break backwards migration, since the destination
won't understand the (unused but still present) xive state it
receives.  So xives can only be created on new machine types.  I'm ok
(at least tentatively) with always creating them on the newer machine
types, regardless of whether the guest ends up exploiting it or not.

> >> Depending on the configuration and
> >> the guest capabilities, the CAS negotiation process will decide which
> >> interrupt model to use, legacy or XIVE.
> >>
> >> The XIVE model makes use of the full range of the IRQ number space
> >> because the IRQ numbers for the CPU IPIs are allocated in the range
> >> below XICS_IRQ_BASE, which is unused by XICS.
> > 
> > Ok.  And I take it 4096 is enough space for the XIVE IPIs for the
> > forseeable future?
> The biggest real system I am aware of as 16 sockets, 192 cores, SMT8. 
> That's 1536 cpus. pseries has a max_cpus of 1024.

Ok, so we can go to double the current system size, but not 4x.  Not
sure if that seems adequate or not.  Still it's a relatively minor

David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!

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