Re: [PATCH 5/5] KVM: x86/pmu: Hide guest counter updates from the VMRUN instruction

[Sean Christopherson <seanjc@google.com>]

On Fri, Mar 10, 2023, Like Xu wrote:
> From: Like Xu <likexu@tencent.com>
> 
> When AMD guest is counting (branch) instructions event, its vPMU should
> first subtract one for any relevant (branch)-instructions enabled counter
> (when it precedes VMRUN and cannot be preempted) to offset the inevitable
> plus-one effect of the VMRUN instruction immediately follows.
> 
> Based on a number of micro observations (also the reason why x86_64/
> pmu_event_filter_test fails on AMD Zen platforms), each VMRUN will
> increment all hw-(branch)-instructions counters by 1, even if they are
> only enabled for guest code. This issue seriously affects the performance
> understanding of guest developers based on (branch) instruction events.
> 
> If the current physical register value on the hardware is ~0x0, it triggers
> an overflow in the guest world right after running VMRUN. Although this
> cannot be avoided on mainstream released hardware, the resulting PMI
> (if configured) will not be incorrectly injected into the guest by vPMU,
> since the delayed injection mechanism for a normal counter overflow
> depends only on the change of pmc->counter values.

IIUC, this is saying that KVM may get a spurious PMI, but otherwise nothing bad
will happen?

> +static inline bool event_is_branch_instruction(struct kvm_pmc *pmc)
> +{
> +	return eventsel_match_perf_hw_id(pmc, PERF_COUNT_HW_INSTRUCTIONS) ||
> +		eventsel_match_perf_hw_id(pmc,
> +					  PERF_COUNT_HW_BRANCH_INSTRUCTIONS);
> +}
> +
> +static inline bool quirky_pmc_will_count_vmrun(struct kvm_pmc *pmc)
> +{
> +	return event_is_branch_instruction(pmc) && event_is_allowed(pmc) &&
> +		!static_call(kvm_x86_get_cpl)(pmc->vcpu);

Wait, really?  VMRUN is counted if and only if it enters to a CPL0 guest?  Can
someone from AMD confirm this?  I was going to say we should just treat this as
"normal" behavior, but counting CPL0 but not CPL>0 is definitely quirky.