Re: [PATCH v3 2/2] cpufreq: add virtual-cpufreq driver

[Saravana Kannan]

On Wed, Aug 2, 2023 at 10:52 PM Viresh Kumar <viresh.kumar@xxxxxxxxxx> wrote:
>
> On 02-08-23, 15:16, Saravana Kannan wrote:
> > This is mainly an optimization to reduce the latency of the "frequency
> > change" which has a huge impact on the performance (as can be seen
> > from the numbers in the cover letter).
> >
> > Setting this flag means that the vCPU thread triggering the MMIO
> > handling (on the host side) is the thread on which the host needs to
> > apply any uclamp settings. So this avoids the VMM having to look up
> > the right vCPU thread that corresponds to this CPU, and any
> > permissions issues wrt setting another threads uclamp, etc. This
> > becomes even more important if/when BPF support is added for handling
> > simple MMIO read/writes. Will Deacon has been working on the eBPF
> > part[1] and IIUC, not setting this flag adds a lot of extra overhead
> > on the BPF side.
> >
> > So, yeah, this flag is very helpful wrt reducing latency/simplifying
> > host side implementation and that's why we want it here.
> >
> > [1] - https://kvm-forum.qemu.org/2023/talk/AZKC77/
>
> Would be good to have a (big) comment in the code explaining that as
> it isn't obvious. Thanks.

Will do.

Thanks,
Saravana