[Devel] [PATCH vz7.6 v2] kvm: x86: hyperv: avoid livelock in oneshot SynIC timers

[Denis V. Lunev]

On 07/24/2017 11:36 AM, Konstantin Khorenko wrote:
> Once more: need this in vz7 update 5 or later?
>

update6, as written in the subject ;)

> -- 
> Best regards,
>
> Konstantin Khorenko,
> Virtuozzo Linux Kernel Team
>
> On 07/24/2017 10:33 AM, Roman Kagan wrote:
>> If the SynIC timer message delivery fails due to SINT message slot being
>> busy, there's no point to attempt starting the timer again until we're
>> notified of the slot being released by the guest (via EOM or EOI).
>>
>> Even worse, when a oneshot timer fails to deliver its message, its
>> re-arming with an expiration time in the past leads to immediate retry
>> of the delivery, and so on, without ever letting the guest vcpu to run
>> and release the slot, which results in a livelock.
>>
>> To avoid that, only start the timer when there's no timer message
>> pending delivery.  When there is, meaning the slot is busy, the
>> processing will be restarted upon notification from the guest that the
>> slot is released.
>>
>> Signed-off-by: Roman Kagan <rkagan at virtuozzo.com>
>> Signed-off-by: Radim Krčmář <rkrcmar at redhat.com>
>> (cherry picked from commit f1ff89ec4447c4e39d275a1ca3de43eed2a92745)
>> ---
>> v1 -> v2:
>>  - actual cherry-pick from mainline kernel
>>
>>  arch/x86/kvm/hyperv.c | 7 ++++---
>>  1 file changed, 4 insertions(+), 3 deletions(-)
>>
>> diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
>> index c8efdce3e702..ab9501c2f32c 100644
>> --- a/arch/x86/kvm/hyperv.c
>> +++ b/arch/x86/kvm/hyperv.c
>> @@ -632,9 +632,10 @@ void kvm_hv_process_stimers(struct kvm_vcpu *vcpu)
>>                  }
>>
>>                  if ((stimer->config & HV_STIMER_ENABLE) &&
>> -                    stimer->count)
>> -                    stimer_start(stimer);
>> -                else
>> +                    stimer->count) {
>> +                    if (!stimer->msg_pending)
>> +                        stimer_start(stimer);
>> +                } else
>>                      stimer_cleanup(stimer);
>>              }
>>          }
>>