Re: [PATCH v2] KVM: s390: Remove false WARN_ON_ONCE for the PQAP instruction

From: Cornelia Huck <cohuck@redhat.com>
To: Christian Borntraeger <borntraeger@de.ibm.com>
Cc: Janosch Frank <frankja@linux.vnet.ibm.com>,
	KVM <kvm@vger.kernel.org>, David Hildenbrand <david@redhat.com>,
	linux-s390 <linux-s390@vger.kernel.org>,
	Qian Cai <cailca@icloud.com>, Pierre Morel <pmorel@linux.ibm.com>,
	Tony Krowiak <akrowiak@linux.ibm.com>
Subject: Re: [PATCH v2] KVM: s390: Remove false WARN_ON_ONCE for the PQAP instruction
Date: Tue, 5 May 2020 10:46:10 +0200	[thread overview]
Message-ID: <20200505104610.469be44e.cohuck@redhat.com> (raw)
In-Reply-To: <20200505083515.2720-1-borntraeger@de.ibm.com>

On Tue,  5 May 2020 10:35:15 +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 as ECA_APIE is an effective bit: If one hypervisor layer has
> turned this bit off, the end result will be that we will get intercepts for
> all function codes. Usually the first one will be a query like PQAP(QCI).
> So the WARN_ON_ONCE is not right. Let us simply remove it.

Thanks, that is helpful to describe.

Fixes: e5282de93105 ("s390: ap: kvm: add PQAP interception for AQIC")

> Cc: Pierre Morel <pmorel@linux.ibm.com>
> Cc: Tony Krowiak <akrowiak@linux.ibm.com>
> Cc: stable@vger.kernel.org
> Link: https://lore.kernel.org/kvm/20200505073525.2287-1-borntraeger@de.ibm.com

This links to v1, which is probably not what you want :)

> Reported-by: Qian Cai <cailca@icloud.com>
> Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
> Reviewed-by: David Hildenbrand <david@redhat.com>
> Reviewed-by: Cornelia Huck <cohuck@redhat.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..893893642415 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 for other
> +	 * function codes, e.g. 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 */