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

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Date:   Mon, 8 Nov 2021 13:50:40 +0100
From:   Christian Borntraeger <borntraeger@...ibm.com>
To:     Janosch Frank <frankja@...ux.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



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).

> 
> @Collin: Could you check that please?