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

Cornelia Huck
2020-05-05T08:29:02Z

On Tue, 5 May 2020 09:55:36 +0200
Christian Borntraeger <borntraeger@de.ibm.com> wrote:

> 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.

Ok, that makes sense, even if it is rather ugly.

With the comment tweaked,
Reviewed-by: Cornelia Huck <cohuck@redhat.com>

> 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 */
> >
>