Re: [PATCH v2] KVM: X86: Emulate APERF/MPERF to report actual vCPU frequency

[Jim Mattson]

On Wed, Dec 22, 2021 at 5:34 AM Like Xu <like.xu.linux@xxxxxxxxx> wrote:
>
> From: Like Xu <likexu@xxxxxxxxxxx>
>
> The aperf/mperf are used to report current CPU frequency after 7d5905dc14a.
> But guest kernel always reports a fixed vCPU frequency in the /proc/cpuinfo,
> which may confuse users especially when turbo is enabled on the host or
> when the vCPU has a noisy high power consumption neighbour task.
>
> Most guests such as Linux will only read accesses to AMPERF msrs, where
> we can passthrough registers to the vcpu as the fast-path (a performance win)
> and once any write accesses are trapped, the emulation will be switched to
> slow-path, which emulates guest APERF/MPERF values based on host values.
> In emulation mode, the returned MPERF msr value will be scaled according
> to the TSCRatio value.
>
> As a minimum effort, KVM exposes the AMPERF feature when the host TSC
> has CONSTANT and NONSTOP features, to avoid the need for more code
> to cover various coner cases coming from host power throttling transitions.
>
> The slow path code reveals an opportunity to refactor update_vcpu_amperf()
> and get_host_amperf() to be more flexible and generic, to cover more
> power-related msrs.
>
> Requested-by: Dongli Cao <caodongli@xxxxxxxxxxxx>
> Requested-by: Li RongQing <lirongqing@xxxxxxxxx>
> Signed-off-by: Like Xu <likexu@xxxxxxxxxxx>

I am not sure that it is necessary for kvm to get involved in the
virtualization of APERF and MPERF at all, and I am highly skeptical of
the need for passing through the hardware MSRs to a guest. Due to
concerns over potential side-channel exploits a la Platypus
(https://platypusattack.com/), we are planning to provide only low
fidelity APERF/MPERF virtualization from userspace, using the
userspace MSR exiting mechanism. Of course, we should be able to do
that whether or not this change goes in, but I was wondering if you
could provide some more details regarding your use case(s).