Re: [PATCH 1/5] KVM: arm64: pvtime: steal-time is only supported when configured

From: Andrew Jones <drjones@redhat.com>
To: Marc Zyngier <maz@kernel.org>
Cc: pbonzini@redhat.com, kvmarm@lists.cs.columbia.edu,
	kvm@vger.kernel.org, steven.price@arm.com
Subject: Re: [PATCH 1/5] KVM: arm64: pvtime: steal-time is only supported when configured
Date: Tue, 28 Jul 2020 14:55:53 +0200	[thread overview]
Message-ID: <20200728125553.3k65bfdxs6u5pb4i@kamzik.brq.redhat.com> (raw)
In-Reply-To: <e2f23a105af0d7cf2daa6f3f8b596177@kernel.org>

On Mon, Jul 27, 2020 at 06:25:50PM +0100, Marc Zyngier wrote:
> Hi Andrew,
> 
> On 2020-07-11 11:04, Andrew Jones wrote:
> > Don't confuse the guest by saying steal-time is supported when
> > it hasn't been configured by userspace and won't work.
> > 
> > Signed-off-by: Andrew Jones <drjones@redhat.com>
> > ---
> >  arch/arm64/kvm/pvtime.c | 5 ++++-
> >  1 file changed, 4 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/arm64/kvm/pvtime.c b/arch/arm64/kvm/pvtime.c
> > index f7b52ce1557e..2b22214909be 100644
> > --- a/arch/arm64/kvm/pvtime.c
> > +++ b/arch/arm64/kvm/pvtime.c
> > @@ -42,9 +42,12 @@ long kvm_hypercall_pv_features(struct kvm_vcpu *vcpu)
> > 
> >  	switch (feature) {
> >  	case ARM_SMCCC_HV_PV_TIME_FEATURES:
> > -	case ARM_SMCCC_HV_PV_TIME_ST:
> >  		val = SMCCC_RET_SUCCESS;
> >  		break;
> > +	case ARM_SMCCC_HV_PV_TIME_ST:
> > +		if (vcpu->arch.steal.base != GPA_INVALID)
> > +			val = SMCCC_RET_SUCCESS;
> > +		break;
> >  	}
> > 
> >  	return val;
> 
> I'm not so sure about this. I have always considered the
> discovery interface to be "do you know about this SMCCC
> function". And if you look at the spec, it says (4.2,
> PV_TIME_FEATURES):
> 
> <quote>
> If PV_call_id identifies PV_TIME_FEATURES, this call returns:
> • NOT_SUPPORTED (-1) to indicate that all
> paravirtualized time functions in this specification are not
> supported.
> • SUCCESS (0) to indicate that all the paravirtualized time
> functions in this specification are supported.
> </quote>
> 
> So the way I understand it, you cannot return "supported"
> for PV_TIME_FEATURES, and yet return NOT_SUPPORTED for
> PV_TIME_ST. It applies to *all* features.
> 
> Yes, this is very bizarre. But I don't think we can deviate
> from it.

Ah, I see your point. But I wonder if we should drop this patch
or if we should change the return of ARM_SMCCC_HV_PV_TIME_FEATURES
to be dependant on all the pv calls?

Discovery would look like this

IF (SMCCC_ARCH_FEATURES, PV_TIME_FEATURES) == 0; THEN
  IF (PV_TIME_FEATURES, PV_TIME_FEATURES) == 0; THEN
    PV_TIME_ST is supported, as well as all other PV calls
  ELIF (PV_TIME_FEATURES, PV_TIME_ST) == 0; THEN
    PV_TIME_ST is supported
  ELIF (PV_TIME_FEATURES, <another-pv-call>) == 0; THEN
    <another-pv-call> is supported
  ...
  ENDIF
ELSE
  No PV calls are supported
ENDIF

I believe the above implements a reasonable interpretation of the
specification, but the all feature (PV_TIME_FEATURES, PV_TIME_FEATURES)
thing is indeed bizarre no matter how you look at it.

Thanks,
drew

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