On 03/07/2016 10:36 PM, Shanker Donthineni wrote: > On 03/03/2016 08:38 AM, Marc Zyngier wrote: >> On 03/03/16 14:26, Shanker Donthineni wrote: >>> On 03/03/2016 08:03 AM, Marc Zyngier wrote: >>>> On 03/03/16 13:25, Shanker Donthineni wrote: >>>>> On 03/02/2016 11:35 AM, Marc Zyngier wrote: >>>>>> On 02/03/16 15:48, Shanker Donthineni wrote: >>>>>> >>>>>>> We haven't started running heavy workloads in VMs. So far we >>>>>>> have noticed this random nature behavior only during guest >>>>>>> kernel boot (at EL1). >>>>>>> >>>>>>> We didn't see this problem on 4.3 kernel. Do you think it is >>>>>>> related to TLB conflicts? >>>>>> I cannot imagine why a DSB would solve a TLB conflict. But the fact >>>>>> that >>>>>> you didn't see it crashing on 4.3 is a good indication that something >>>>>> else it at play. >>>>>> >>>>>> In 4.5, we've rewritten a large part of KVM in C, which has changed the >>>>>> ordering of the various accesses a lot. It could be that a latent >>>>>> problem is now exposed more widely. >>>>>> >>>>>> Can you try moving this DSB around and find out what is the earliest >>>>>> point where it solves this problem? Some sort of bisection? >>>>> The maximum I can move up 'dsb ishst' to the beginning of >>>>> __guest_enter() but not out side of this function. >>>>> >>>>> I don't understand why it is failing below code, branch >>>>> instruction causing problems. >>>>> >>>>> /* Jump in the fire! */ >>>>> + dsb(ishst); >>>>> exit_code = __guest_enter(vcpu, host_ctxt); >>>>> /* And we're baaack! */ >>>> That's very worrying. I can't see how the branch can have an influence >>>> on the the DSB (nor why the DSB has an influence on the rest of the >>>> execution, btw). >>>> >>>> What if you replace the DSB with an ISB? Do you observe a similar >>>> behaviour (works if the barrier is in __guest_enter, but not if it is >>>> outside)? >>> I have already tried with isb without success. I did another >>> experiment flush stage-2 TLBs before calling __guest_enetr(), >>> it fixed the problem. >> I suspected something like that. But it is such a massive hammer that it >> will hide any sort of subtle bug (HW *and* SW). >> >>>> Another thing worth looking at is what happened just before we decided >>>> to get back into the guest. Or to put it differently, what was the >>>> reason to exit the first place. Was it a Stage-2 fault by any chance? >>> I will collect as much possible debug data and update results >>> to you. I went through your KVM refracted 'C' code and did not >>> find any thing suspicious. I am thinking may be Qualcomm CPUs >>> have a very aggressive prefech logic that causing the problem. >> OK. Please keep me posted about your findings. Also maybe involving some >> HW people ouwld be a good idea (running something in an emulator, for >> example...). This has been confirmed to be a hardware defect with a firmware workaround. Regards, Christopher Covington -- Qualcomm Innovation Center, Inc. Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project _______________________________________________ kvmarm mailing list kvmarm@xxxxxxxxxxxxxxxxxxxxx https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
