Re: [Qemu-devel] [Qemu-ppc] [PATCH 5/5] target-ppc: Handle cases when multi-processors get machine-check

[Alexander Graf <agraf@suse.de>]

On 28.08.14 08:56, Aravinda Prasad wrote:
> 
> 
> On Wednesday 27 August 2014 04:10 PM, Alexander Graf wrote:
>>
>>
>> On 25.08.14 15:45, Aravinda Prasad wrote:
>>> It is possible for multi-processors to experience machine
>>> check at or about the same time. As per PAPR, subsequent
>>> processors serialize waiting for the first processor to
>>> issue the ibm,nmi-interlock call.
>>>
>>> The second processor retries if the first processor which
>>> received a machine check is still reading the error log
>>> and is yet to issue ibm,nmi-interlock call.
>>>
>>> This patch implements this functionality.
>>>
>>> Signed-off-by: Aravinda Prasad <aravinda@linux.vnet.ibm.com>
>>
>> This patch doesn't make any sense. Both threads will issue an HCALL
>> which will get locked inside of QEMU, so we'll never see the case where
>> both hypercalls get processed at the same time.
> 
> AFAIK, only one thread can succeed entering qemu upon parallel hcall
> from different guest CPUs as it is gated by a lock. Hence one hcall is
> processed at a time.
> 
> As per PAPR, we don't want any other KVMPPC_H_REPORT_ERR hcall to be
> processed at the same time and further KVMPPC_H_REPORT_ERR hcall thus
> issued should wait until the OS issues ibm,nmi-interlock.

Oh, now I understand. The locking time is from
[h_report_mc_err...rtas_ibm_nmi_interlock].

This should definitely go into the comment on the check in
h_report_mc_err. In fact, remove the fact that only one thread can
execute and instead write where the lock gets unset and that during that
phase only one vcpu may process the NMI.


Alex