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

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Message-ID: <CAOnJCUKZV+0Xts6C4QY7X+Wak0ZR_f8wPtEAtH4PEmh2-_AcWw@mail.gmail.com>
Date:   Thu, 24 Nov 2022 02:18:26 -0800
From:   Atish Patra <atishp@...shpatra.org>
To:     Andrew Jones <ajones@...tanamicro.com>
Cc:     Atish Patra <atishp@...osinc.com>, linux-kernel@...r.kernel.org,
        Albert Ou <aou@...s.berkeley.edu>,
        Anup Patel <anup@...infault.org>, Guo Ren <guoren@...nel.org>,
        kvm-riscv@...ts.infradead.org, kvm@...r.kernel.org,
        linux-riscv@...ts.infradead.org,
        Mark Rutland <mark.rutland@....com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Will Deacon <will@...nel.org>
Subject: Re: [RFC 6/9] RISC-V: KVM: Add SBI PMU extension support

On Wed, Nov 23, 2022 at 5:58 AM Andrew Jones <ajones@...tanamicro.com> wrote:
>
> 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@...tanamicro.com> 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.
>

Sure. Will add those 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.
>

How does it handle hetergenous systems in per 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.
>

Do we really need a per VM interface ? I was thinking we can just
continue to use
one reg interface for PMU as well. We probably need two of them.

1. To enable/disable SBI extension
    -- The probe function will depend on this
2. PMU specific get/set
    -- Number of hpmcounters
    -- hpmcounter width
    -- enable PMU


> Thanks,
> drew



-- 
Regards,
Atish