Re: [PATCH 2/2] apic: Use 32bit APIC ID for migration instance ID

[Dr. David Alan Gilbert]

* Peter Xu (address@hidden) wrote:
> Migration is silently broken now with x2apic config like this:
> 
>      -smp 200,maxcpus=288,sockets=2,cores=72,threads=2 \
>      -device intel-iommu,intremap=on,eim=on
> 
> After migration, the guest kernel could hang at anything, due to
> x2apic bit not migrated correctly in IA32_APIC_BASE on some vcpus, so
> any operations related to x2apic could be broken then (e.g., RDMSR on
> x2apic MSRs could fail because KVM would think that the vcpu hasn't
> enabled x2apic at all).
> 
> The issue is that the x2apic bit was never applied correctly for vcpus
> whose ID > 255 when migrate completes, and that's because when we
> migrate APIC we use the APICCommonState.id as instance ID of the
> migration stream, while that's too short for x2apic.
> 
> Let's use the newly introduced initial_apic_id for that.

I'd like to understand a few things:
   a) Does this change the instance ID of existing APICs on the
migration stream? 
     a1) Ever for <256 CPUs?
     a2) For >=256 CPUs?

    [Because changing the ID breaks migration]

  b) Is the instance ID constant - I can see it's a property on the
     APIC, but I cna't see who sets it

  c) In the case where it fails, did we end up registering two
     devices with the same name and instance ID?  If so, is it worth
     adding a check that would error if we tried?

Dave

> 
> Signed-off-by: Peter Xu <address@hidden>
> ---
>  hw/intc/apic_common.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/hw/intc/apic_common.c b/hw/intc/apic_common.c
> index aafd8e0e33..6024a3e06a 100644
> --- a/hw/intc/apic_common.c
> +++ b/hw/intc/apic_common.c
> @@ -315,7 +315,7 @@ static void apic_common_realize(DeviceState *dev, Error 
> **errp)
>      APICCommonState *s = APIC_COMMON(dev);
>      APICCommonClass *info;
>      static DeviceState *vapic;
> -    int instance_id = s->id;
> +    int64_t instance_id = s->initial_apic_id;
>  
>      info = APIC_COMMON_GET_CLASS(s);
>      info->realize(dev, errp);
> -- 
> 2.21.0
> 
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK