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

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Date:   Mon, 8 Nov 2021 13:48:27 +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 13:04, 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.

@Collin: Could you check that please?