Re: [PATCH 3/3] hw/arm/virt: Implement kvm-steal-time

[Andrew Jones]

On Tue, Jul 21, 2020 at 11:46:12AM +0100, Peter Maydell wrote:
> > +    if (!probed) {
> > +        probed = true;
> > +        if (kvm_check_extension(kvm_state, KVM_CAP_VCPU_ATTRIBUTES)) {
> > +            if (!kvm_arm_create_scratch_host_vcpu(NULL, fdarray, NULL)) {
> > +                error_report("Failed to create scratch VCPU");
> > +                abort();
> > +            }
> > +
> > +            has_steal_time = kvm_device_check_attr(fdarray[2],
> > +                                                   
> > KVM_ARM_VCPU_PVTIME_CTRL,
> > +                                                   
> > KVM_ARM_VCPU_PVTIME_IPA);
> > +
> > +            kvm_arm_destroy_scratch_host_vcpu(fdarray);
> 
> I was a bit surprised that we create a scratch VCPU here, but
> it looks like we've opted for "create scratch VCPU, check specific
> detail, destroy VCPU" as the usual coding pattern rather than trying
> to coalesce into a single "create scratch VCPU once, cache all
> the info we might need for later". I guess if somebody (a) cares
> about startup performance and (b) finds through profiling that
> creation-and-destruction of the scratch VMs/VCPUs is a significant
> contributor they can write the refactoring themselves :-)

There's still a chance I'll be changing this to a KVM CAP if the KVM
maintainers accept the patch I proposed to add one.

> 
> > +        }
> > +    }
> > +
> > +    if (cpu->kvm_steal_time == ON_OFF_AUTO_AUTO) {
> > +        if (!has_steal_time || !arm_feature(&cpu->env, 
> > ARM_FEATURE_AARCH64)) {
> > +            cpu->kvm_steal_time = ON_OFF_AUTO_OFF;
> > +        } else {
> > +            cpu->kvm_steal_time = ON_OFF_AUTO_ON;
> > +        }
> > +    } else if (cpu->kvm_steal_time == ON_OFF_AUTO_ON) {
> > +        if (!has_steal_time) {
> > +            error_setg(errp, "'kvm-steal-time' cannot be enabled "
> > +                             "on this host");
> > +            return;
> > +        } else if (!arm_feature(&cpu->env, ARM_FEATURE_AARCH64)) {
> > +            error_setg(errp, "'kvm-steal-time' cannot be enabled "
> > +                             "for AArch32 guests");
> 
> Why not? Unlike aarch32-host KVM, aarch32-guest KVM is
> still supported. What's the missing piece for kvm-steal-time
> to work in that setup?

The specification. DEN0057A chapter 2 says "This specification only covers
systems in which the Execution state of the hypervisor as well as EL1 of
virtual machines is AArch64.". And, to ensure that the smc/hvc calls are
only specified as smc64/hvc64. I find that a bit disappointing, since
there's nothing about steal-time that should be 64-bit specific, but
that's how this cookie is crumbling...

I'll add a comment to explain this error for v2.

> 
> > +            return;
> > +        }
> > +    }
> > +}
> > +
> >  bool kvm_arm_aarch32_supported(void)
> >  {
> >      return kvm_check_extension(kvm_state, KVM_CAP_ARM_EL1_32BIT);
> 
> >  static inline void kvm_arm_add_vcpu_properties(Object *obj) {}
> > +static inline void kvm_arm_steal_time_finalize(ARMCPU *cpu, Error **errp) 
> > {}
> 
> Does this stub need to report an error to the caller via errp,
> or is it a "never called but needs to exist to avoid linker errors" ?

The second one, as we can't have kvm_enabled() and !defined(CONFIG_KVM).
Hmm, these types of stubs would be more robust to refactoring if we put
build bugs in them. I can try to analyze all the stubs in this #else to
see which ones should be returning false/error/nothing vs. build bugging.

Thanks,
drew