Re: [RFC PATCH v3 00/58] Mediated Passthrough vPMU 3.0 for x86

[Sean Christopherson <seanjc@google.com>]

On Thu, Aug 01, 2024, Mingwei Zhang wrote:
> This series contains perf interface improvements to address Peter's
> comments. In addition, fix several bugs for v2. This version is based on
> 6.10-rc4. The main changes are:
> 
>  - Use atomics to replace refcounts to track the nr_mediated_pmu_vms.
>  - Use the generic ctx_sched_{in,out}() to switch PMU resources when a
>    guest is entering and exiting.
>  - Add a new EVENT_GUEST flag to indicate the context switch case of
>    entering and exiting a guest. Updates the generic ctx_sched_{in,out}
>    to specifically handle this case, especially for time management.
>  - Switch PMI vector in perf_guest_{enter,exit}() as well. Add a new
>    driver-specific interface to facilitate the switch.
>  - Remove the PMU_FL_PASSTHROUGH flag and uses the PASSTHROUGH pmu
>    capability instead.
>  - Adjust commit sequence in PERF and KVM PMI interrupt functions.
>  - Use pmc_is_globally_enabled() check in emulated counter increment [1]
>  - Fix PMU context switch [2] by using rdpmc() instead of rdmsr().
> 
> AMD fixes:
>  - Add support for legacy PMU MSRs in MSR interception.
>  - Make MSR usage consistent if PerfMonV2 is available.
>  - Avoid enabling passthrough vPMU when local APIC is not in kernel.
>  - increment counters in emulation mode.
> 
> This series is organized in the following order:
> 
> Patches 1-3:
>  - Immediate bug fixes that can be applied to Linux tip.
>  - Note: will put immediate fixes ahead in the future. These patches
>    might be duplicated with existing posts.
>  - Note: patches 1-2 are needed for AMD when host kernel enables
>    preemption. Otherwise, guest will suffer from softlockup.
> 
> Patches 4-17:
>  - Perf side changes, infra changes in core pmu with API for KVM.
> 
> Patches 18-48:
>  - KVM mediated passthrough vPMU framework + Intel CPU implementation.
> 
> Patches 49-58:
>  - AMD CPU implementation for vPMU.

Please rename everything in KVM to drop "passthrough" and simply use "mediated"
for the overall concept.  This is not a passthrough setup by any stretch of the
word.  I realize it's a ton of renaming, but calling this "passthrough" is very
misleading and actively harmful for unsuspecting readers.

For helpers and/or comments that deal with intercepting (or not) MSRs, use
"intercept" and appropriate variations.  E.g. intel_pmu_update_msr_intercepts().

And for RDPMC, maybe kvm_rdpmc_in_guest() to follow kvm_{hlt,mwait,pause,cstate_in_guest()?
I don't love the terminology, but there's a lot of value in being consistent
throughout KVM.

I am not willing to budge on this, at all.

I'm ok with the perf side of things using "passthrough" if "mediated" feels weird
in that context and we can't come up with a better option, but for the KVM side,
"passthrough" is simply wrong.