Re: [PATCH 5/5] KVM: PPC: Book3S HV: Check wait conditions before sleeping in kvmppc_vcore_blocked

[Alexander Graf <agraf@suse.de>]

On 20.11.14 20:31, Suresh E. Warrier wrote:
> 
> 
> On 11/20/2014 11:36 AM, Alexander Graf wrote:
>>
>>
>> On 03.11.14 05:52, Paul Mackerras wrote:
>>> From: "Suresh E. Warrier" <warrier@linux.vnet.ibm.com>
>>>
>>> The kvmppc_vcore_blocked() code does not check for the wait condition
>>> after putting the process on the wait queue. This means that it is
>>> possible for an external interrupt to become pending, but the vcpu to
>>> remain asleep until the next decrementer interrupt.  The fix is to
>>> make one last check for pending exceptions and ceded state before
>>> calling schedule().
>>>
>>> Signed-off-by: Suresh Warrier <warrier@linux.vnet.ibm.com>
>>> Signed-off-by: Paul Mackerras <paulus@samba.org>
>>
>> I don't understand the race you're fixing here. Can you please explain it?
>>
> 
> When a virtual interrupt needs to be delivered to the guest, and the
> virtual ICS state for the interrupt and virtual ICP state for the VCPU
> allow for the VCPU to be immediately interrupted, we
> 1. Set the BOOK3S_INTERRUPT_EXTERNAL_LEVEL bit in pending_exceptions.
> 2. Call kvmppc_fast_vcpu_kick_hv(), which checks the wait queue at vcpu->wq
>    to wake the VCPU up.
> 
> The caller of kvmppc_vcore_blocked() does the check for pending exceptions, but
> there is a race condition here and we do need to check again after the VCPU
> is put on the wait queue.

I see. Thanks, applied to kvm-ppc-queue.


Alex