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

[Anup Patel]

On Thu, Nov 24, 2022 at 4:21 PM Andrew Jones <ajones@xxxxxxxxxxxxxxxx> wrote:
>
> On Thu, Nov 24, 2022 at 02:18:26AM -0800, Atish Patra wrote:
> > On Wed, Nov 23, 2022 at 5:58 AM Andrew Jones <ajones@xxxxxxxxxxxxxxxx> wrote:
> > >
> > > On Tue, Nov 22, 2022 at 03:08:34PM -0800, Atish Patra wrote:
> ...
> > > > 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 ?
>
> I don't think it does, but neither does arm64. Afaik, the only way to run
> KVM VMs on heterogeneous systems is to pin the VM to one set of the CPUs,
> i.e. make sure the system it runs on is homogeneous.
>
> I agree we shouldn't paint ourselves into a homogeneous-only corner for
> riscv, though, so if it's possible to use VCPU APIs, then I guess we
> should. Although, one thing to keep in mind is that if the same ioctl
> needs to be run on each VCPU, then, when we start building VMs with
> hundreds of VCPUs, we'll see slow VM starts.
>
> >
> > > >
> > > > 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
>
> ONE_REG is good for registers and virtual registers, which means the
> number of hpmcounters and the hpmcounter width are probably good
> candidates, but I'm not sure we should use it for enable/init types of
> purposes.

We are already using ONE_REG interface to enable/disable
ISA extensions so we should follow the same pattern and have
ONE_REG interface to enable/disable SBI extensions as well.

Regards,
Anup