Re: [Qemu-devel] [Qemu-ppc] [PATCH for-2.2] spapr: add host Linux version information to device tree

[Alexander Graf <agraf@suse.de>]

On 18.07.14 13:59, Eric Blake wrote:
> On 07/17/2014 10:31 PM, cyrilbur@gmail.com wrote:
>> It may prove useful know which Linux distribution version the host machine
>> is running when an issue in the guest arises but a user cannot access
>> the host.
>>
>> Signed-off-by: Cyril Bur <cyril.bur@au1.ibm.com>
>> ---
>>   hw/ppc/spapr.c       |  8 +++++++
>>   target-ppc/kvm.c     | 62 ++++++++++++++++++++++++++++++++++++++++++++++++++++
>>   target-ppc/kvm_ppc.h |  6 +++++
>>   3 files changed, 76 insertions(+)
>>
>> diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
>> index 6b48a26..391d47a 100644
>> --- a/hw/ppc/spapr.c
>> +++ b/hw/ppc/spapr.c
>> @@ -375,6 +375,14 @@ static void *spapr_create_fdt_skel(hwaddr initrd_base,
>>       _FDT((fdt_property_string(fdt, "vm,uuid", buf)));
>>       g_free(buf);
>>   
>> +    /*
>> +     * Add info to the guest FDT to tell it what linux the host is
>> +     */
>> +    if (kvmppc_get_linux_host(&buf)) {
>> +        _FDT((fdt_property_string(fdt, "linux,host", buf)));
>> +        g_free(buf);
>> +    }
> Ouch.  What does this do for migration?  By exposing it to the guest,
> you have made it part of the guest ABI, and now you have limited
> yourself to migrate only when the destination host is identical to the
> source if you don't want to risk breaking the guest.  Without this, it
> seems feasible to migrate from a machine on an older host to another
> machine on a newer host.

sPAPR has a mechanism to inject a "post-migration" interrupt into the 
guest and provide a device tree diff. That way we can change these host 
specific values on the fly after migration happened.

Maybe it makes sense to hold off these host specific device tree chunks 
until after we have that mechanism implemented though.


Alex