Re: [Qemu-devel] [PULL 19/48] spapr: allocate the ICPState object from under sPAPRCPUCore

[David Gibson <david@gibson.dropbear.id.au>]

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On Wed, May 17, 2017 at 07:50:42AM +0200, Cédric Le Goater wrote:
> On 05/16/2017 06:10 PM, Greg Kurz wrote:
> > On Tue, 16 May 2017 17:18:27 +0200
> > Cédric Le Goater <clg@kaod.org> wrote:
> > 
> >> On 05/16/2017 02:55 PM, Laurent Vivier wrote:
> >>> On 16/05/2017 14:50, Cédric Le Goater wrote:  
> >>>> On 05/16/2017 02:03 PM, Laurent Vivier wrote:  
> >>>>> On 26/04/2017 09:00, David Gibson wrote:  
> >>>>>> From: Cédric Le Goater <clg@kaod.org>
> >>>>>>
> >>>>>> Today, all the ICPs are created before the CPUs, stored in an array
> >>>>>> under the sPAPR machine and linked to the CPU when the core threads
> >>>>>> are realized. This modeling brings some complexity when a lookup in
> >>>>>> the array is required and it can be simplified by allocating the ICPs
> >>>>>> when the CPUs are.
> >>>>>>
> >>>>>> This is the purpose of this proposal which introduces a new 'icp_type'
> >>>>>> field under the machine and creates the ICP objects of the right type
> >>>>>> (KVM or not) before the PowerPCCPU object are.
> >>>>>>
> >>>>>> This change allows more cleanups : the removal of the icps array under
> >>>>>> the sPAPR machine and the removal of the xics_get_cpu_index_by_dt_id()
> >>>>>> helper.
> >>>>>>
> >>>>>> Signed-off-by: Cédric Le Goater <clg@kaod.org>
> >>>>>> Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
> >>>>>> Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> >>>>>> ---
> >>>>>>  hw/intc/xics.c          | 11 -----------
> >>>>>>  hw/ppc/spapr.c          | 47 ++++++++++++++---------------------------------
> >>>>>>  hw/ppc/spapr_cpu_core.c | 18 ++++++++++++++----
> >>>>>>  include/hw/ppc/spapr.h  |  2 +-
> >>>>>>  include/hw/ppc/xics.h   |  2 --
> >>>>>>  5 files changed, 29 insertions(+), 51 deletions(-)
> >>>>>>  
> >>>>>
> >>>>> This commit breaks CPU re-hotplugging with KVM
> >>>>>
> >>>>> the sequence "device_add, device_del, device_add" brings to the
> >>>>> following error message:
> >>>>>
> >>>>>     Unable to connect CPUx to kernel XICS: Device or resource busy
> >>>>>
> >>>>> It comes from icp_kvm_cpu_setup():
> >>>>>
> >>>>> ...
> >>>>>     ret = kvm_vcpu_enable_cap(cs, KVM_CAP_IRQ_XICS, 0, kernel_xics_fd,
> >>>>>                               kvm_arch_vcpu_id(cs));
> >>>>>     if (ret < 0) {
> >>>>>         error_report("Unable to connect CPU%ld to kernel XICS: %s",
> >>>>>                      kvm_arch_vcpu_id(cs), strerror(errno));
> >>>>>         exit(1);
> >>>>>     }
> >>>>> ..
> >>>>>
> >>>>> It should be protected by cap_irq_xics_enabled:
> >>>>>
> >>>>> ...
> >>>>>     /*
> >>>>>      * If we are reusing a parked vCPU fd corresponding to the CPU
> >>>>>      * which was hot-removed earlier we don't have to renable
> >>>>>      * KVM_CAP_IRQ_XICS capability again.
> >>>>>      */
> >>>>>     if (icp->cap_irq_xics_enabled) {
> >>>>>         return;
> >>>>>     }
> >>>>>
> >>>>> ...
> >>>>>     ret = kvm_vcpu_enable_cap(...);
> >>>>> ...
> >>>>>     icp->cap_irq_xics_enabled = true;
> >>>>> ...
> >>>>>
> >>>>> But since this commit, "icp" is a new object on each call:
> >>>>>
> >>>>> spapr_cpu_core_realize_child()
> >>>>> ...
> >>>>>     obj = object_new(spapr->icp_type);
> >>>>> ...
> >>>>>     xics_cpu_setup(XICS_FABRIC(spapr), cpu, ICP(obj));
> >>>>>     ...
> >>>>>             icpc->cpu_setup(icp, cpu); -> icp_kvm_cpu_setup()
> >>>>>     ...
> >>>>> ...
> >>>>>
> >>>>> and "cap_irq_xics_enabled" is reinitialized.
> >>>>>
> >>>>> Any idea how to fix that?  
> >>>>
> >>>> it seems that a cleanup is not done in the kernel. We are missing
> >>>> a way to call kvmppc_xics_free_icp() from QEMU. Today the only
> >>>> way is to destroy the vcpu.   
> >>>
> >>> The commit introducing this hack, for reference:
> >>>
> >>> commit a45863bda90daa8ec39e5a312b9734fd4665b016
> >>> Author: Bharata B Rao <bharata@linux.vnet.ibm.com>
> >>> Date:   Thu Jul 2 16:23:20 2015 +1000
> >>>
> >>>     xics_kvm: Don't enable KVM_CAP_IRQ_XICS if already enabled
> >>>     
> >>>     When supporting CPU hot removal by parking the vCPU fd and reusing
> >>>     it during hotplug again, there can be cases where we try to reenable
> >>>     KVM_CAP_IRQ_XICS CAP for the vCPU for which it was already enabled.
> >>>     Introduce a boolean member in ICPState to track this and don't
> >>>     reenable the CAP if it was already enabled earlier.
> >>>     
> >>>     Re-enabling this CAP should ideally work, but currently it results in
> >>>     kernel trying to create and associate ICP with this vCPU and that
> >>>     fails since there is already an ICP associated with it. Hence this
> >>>     patch is needed to work around this problem in the kernel.
> >>>     
> >>>     This change allows CPU hot removal to work for sPAPR.
> >>>     
> >>>     Signed-off-by: Bharata B Rao <bharata@linux.vnet.ibm.com>
> >>>     Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
> >>>     Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> >>>     Signed-off-by: Alexander Graf <agraf@suse.de>  
> >>
> >> OK. 
> >>
> >> Greg is looking at re-adding the ICPState array because of a 
> >> migration issue with older machines. We might need to do so 
> >> unconditionally ...
> >>
> > 
> > That would be a pity to carry on with the pre-allocated ICPStates for
> > new machine types just because of that... What about keeping track
> > of all the cap_irq_xics_enabled flags in a separate max_cpus sized
> > static array ?
> 
> Could we use 'cpu->unplug' instead ?

I've only half followed this discussion, but fwiw I prefer the idea of
"parking" in-kernel ICP objects, similarly to the way we do for
removed VCPUs, rather than going back to keeping ICP objects around
indefinitely and unconditionally.

-- 
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_!
http://www.ozlabs.org/~dgibson

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