linux-kernel - Re: [PATCH] KVM: s390x: add debug statement for diag 318 CPNC data

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Message-ID: <446e3ace-16e6-0cc4-874f-7a5caa46d3c1@linux.ibm.com>
Date:   Mon, 8 Nov 2021 14:04:12 +0100
From:   Janosch Frank <frankja@...ux.ibm.com>
To:     Christian Borntraeger <borntraeger@...ibm.com>,
        Collin Walling <walling@...ux.ibm.com>,
        linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     david@...hat.com, imbrenda@...ux.ibm.com
Subject: Re: [PATCH] KVM: s390x: add debug statement for diag 318 CPNC data

On 11/8/21 13:50, Christian Borntraeger wrote:
> 
> 
> Am 08.11.21 um 13:48 schrieb Janosch Frank:
>> On 11/8/21 13:04, Christian Borntraeger wrote:
>>>
>>>
>>> Am 08.11.21 um 12:12 schrieb Janosch Frank:
>>>> On 10/27/21 04:54, Collin Walling wrote:
>>>>> The diag 318 data contains values that denote information regarding the
>>>>> guest's environment. Currently, it is unecessarily difficult to observe
>>>>> this value (either manually-inserted debug statements, gdb stepping, mem
>>>>> dumping etc). It's useful to observe this information to obtain an
>>>>> at-a-glance view of the guest's environment, so lets add a simple VCPU
>>>>> event that prints the CPNC to the s390dbf logs.
>>>>>
>>>>> Signed-off-by: Collin Walling <walling@...ux.ibm.com>
>>>>> ---
>>>>>     arch/s390/kvm/kvm-s390.c | 1 +
>>>>>     1 file changed, 1 insertion(+)
>>>>>
>>>>> diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
>>>>> index 6a6dd5e1daf6..da3ff24eabd0 100644
>>>>> --- a/arch/s390/kvm/kvm-s390.c
>>>>> +++ b/arch/s390/kvm/kvm-s390.c
>>>>> @@ -4254,6 +4254,7 @@ static void sync_regs_fmt2(struct kvm_vcpu *vcpu)
>>>>>         if (kvm_run->kvm_dirty_regs & KVM_SYNC_DIAG318) {
>>>>>             vcpu->arch.diag318_info.val = kvm_run->s.regs.diag318;
>>>>>             vcpu->arch.sie_block->cpnc = vcpu->arch.diag318_info.cpnc;
>>>>> +        VCPU_EVENT(vcpu, 2, "setting cpnc to %d", vcpu->arch.diag318_info.cpnc);
>>>>>         }
>>>>>         /*
>>>>>          * If userspace sets the riccb (e.g. after migration) to a valid state,
>>>>>
>>>>
>>>> Won't that turn up for every vcpu and spam the log?
>>>
>>> only if the userspace always sets the dirty bit (which it should not).
>>>
>>
>> But that's exactly what it does, no?
>> We do a loop over all vcpus and call kvm_s390_set_diag318() which sets the info in kvm_run and sets the diag318 bit in the kvm_dirty_regs.
> 
> Yes, ONCE per CPU. And this is exactly what I want to see. (and it did show a bug in qemu that we only set it for one cpu to the correct value).

Ok.
I didn't really want to have n entries in the log hence I asked.

318 is a bit weird as it's a per VM value we need to put into all sie 
blocks.

> 
>>
>> @Collin: Could you check that please?