On 2020-07-28 13:55, Andrew Jones wrote:
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.
It it indeed true to the spec. Thankfully we only support PV_TIME
as a feature for now, so we are (sort of) immune to the braindead
aspect of the discovery protocol.
I think returning a failure when PV_TIME isn't setup is a valid thing
to do, as long as it applies to all functions (i.e. something like
the below patch).
Thanks,
M.
diff --git a/arch/arm64/kvm/pvtime.c b/arch/arm64/kvm/pvtime.c
index f7b52ce1557e..c3ef4ebd6846 100644
--- a/arch/arm64/kvm/pvtime.c
+++ b/arch/arm64/kvm/pvtime.c
@@ -43,7 +43,8 @@ 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;
+ if (vcpu->arch.steal.base != GPA_INVALID)
+ val = SMCCC_RET_SUCCESS;
break;
}
--
Jazz is not dead. It just smells funny...
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