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@xxxxxxxxxx>
> > ---
> > 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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