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

[Cornelia Huck <cohuck@redhat.com>]

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?

> 
> 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 */