Re: [RFC 6/9] RISC-V: KVM: Add SBI PMU extension support

[Andrew Jones]

On Tue, Nov 22, 2022 at 03:08:34PM -0800, Atish Patra wrote:
> On Tue, Nov 1, 2022 at 7:26 AM Andrew Jones <ajones@xxxxxxxxxxxxxxxx> wrote:
> >
> > On Mon, Jul 18, 2022 at 10:02:02AM -0700, Atish Patra wrote:
...
> > > +static int kvm_sbi_ext_pmu_handler(struct kvm_vcpu *vcpu, struct kvm_run *run,
> > > +                                unsigned long *out_val,
> > > +                                struct kvm_cpu_trap *utrap,
> > > +                                bool *exit)
> > > +{
> > > +     int ret = -EOPNOTSUPP;
> > > +     struct kvm_cpu_context *cp = &vcpu->arch.guest_context;
> > > +     unsigned long funcid = cp->a6;
> > > +     uint64_t temp;
> >
> > I think we need something like
> >
> >    if (!vcpu_to_pmu(vcpu)->enabled)
> >       return -EOPNOTSUPP;
> >
> > here. Where 'enabled' is only true when we successfully initialize
> > the pmu. And, successful initialization depends on
> 
> Yes. I have added the flag. But should we do the check here or
> respective function
> as a paranoia sanity check ?

I think something like above that returns a not-supported error should be
in all the entry points, like the top level SBI call handler. Functions
that should never be called unless the PMU is active could have WARNs
added for sanity checks.

> 
> > IS_ENABLED(CONFIG_RISCV_PMU_SBI) and
> 
> Why do we need to guard under CONFIG_RISCV_PMU_SBI ?
> vcpu_sbi_pmu.c is only compiled under CONFIG_RISCV_PMU_SBI
> 
> If CONFIG_RISCV_PMU_SBI is not enabled, probe function will return failure.

You're right. We don't need explicit config checks for things that can't
exist without the config.

> 
> In fact, I think we should also add the pmu enabled check in the probe function
> itself. Probe function(kvm_sbi_ext_pmu_probe) should only true when
> both vcpu_to_pmu(vcpu)->enabled and
> riscv_isa_extension_available(NULL, SSCOFPMF) are true.
> 
> Thoughts?

Agreed.

> 
> > riscv_isa_extension_available(NULL, SSCOFPMF) as well as not
> > failing other things. And, KVM userspace should be able to
> > disable the pmu, which keep enabled from being set as well.
> >
> We already have provisions for disabling sscofpmf from guests via ISA
> one reg interface.
> Do you mean disable the entire PMU from userspace ?

Yes. We may need to configure a VM without a PMU to increase its
migratability, workaround errata, or just for testing/debugging purposes.

> 
> Currently, ARM64 enables pmu from user space using device control APIs
> on vcpu fd.
> Are you suggesting we should do something like that ?

Yes. Although choosing which KVM API should be used could probably be
thought-out again. x86 uses VM ioctls.

> 
> If PMU needs to have device control APIs (either via vcpu fd or its
> own), we can retrieve
> the hpmcounter width and count from there as well.

Right. We need to decide how the VM/VCPU + PMU user interface should look.
A separate PMU device, like arm64 has, sounds good, but the ioctl
sequences for initialization may get more tricky.

Thanks,
drew