Re: [PATCH] KVM: s390: Remove false WARN_ON_ONCE for the PQAP instruction

Christian Borntraeger
2020-05-05T07:55:43Z

On 05.05.20 09:53, Cornelia Huck wrote:
> On Tue, 5 May 2020 09:35:25 +0200
> Christian Borntraeger <borntraeger@de.ibm.com> wrote:
>
>> In LPAR we will only get an intercept for FC==3 for the PQAP
>> instruction. Running nested under z/VM can result in other intercepts as
>> well, for example PQAP(QCI). So the WARN_ON_ONCE is not right. Let
>> us simply remove it.
>
> While I agree with removing the WARN_ON_ONCE, I'm wondering why z/VM
> gives us intercepts for those fcs... is that just a result of nesting
> (or the z/VM implementation), or is there anything we might want to do?

Yes nesting.
The ECA bit for interpretion is an effective one. So if the ECA bit is off
in z/VM (no crypto cards) our ECA bit is basically ignored as these bits
are ANDed.
I asked Tony to ask the z/VM team if that is the case here.

>
>>
>> Cc: Pierre Morel <pmorel@linux.ibm.com>
>> Cc: Tony Krowiak <akrowiak@linux.ibm.com>
>> Reported-by: Qian Cai <cailca@icloud.com>
>> Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
>> ---
>> arch/s390/kvm/priv.c | 4 +++-
>> 1 file changed, 3 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/s390/kvm/priv.c b/arch/s390/kvm/priv.c
>> index 69a824f9ef0b..bbe46c6aedbf 100644
>> --- a/arch/s390/kvm/priv.c
>> +++ b/arch/s390/kvm/priv.c
>> @@ -626,10 +626,12 @@ static int handle_pqap(struct kvm_vcpu *vcpu)
>> * available for the guest are AQIC and TAPQ with the t bit set
>> * since we do not set IC.3 (FIII) we currently will only intercept
>> * the AQIC function code.
>> + * Note: running nested under z/VM can result in intercepts, e.g.
>
> s/intercepts/intercepts for other function codes/
>
>> + * for PQAP(QCI). We do not support this and bail out.
>> */
>> reg0 = vcpu->run->s.regs.gprs[0];
>> fc = (reg0 >> 24) & 0xff;
>> - if (WARN_ON_ONCE(fc != 0x03))
>> + if (fc != 0x03)
>> return -EOPNOTSUPP;
>>
>> /* PQAP instruction is allowed for guest kernel only */
>