Re: [PATCH v2] KVM: x86/intr: Explicitly check NMI from guest to eliminate false positives

[Dongli Zhang]

Hi Like,

On 12/5/23 19:20, Like Xu wrote:
> From: Like Xu <likexu@xxxxxxxxxxx>
> 
> Explicitly checking the source of external interrupt is indeed NMI and not
> other types in the kvm_arch_pmi_in_guest(), which prevents perf-kvm false
> positive samples generated in perf/core NMI mode after vm-exit but before
> kvm_before_interrupt() from being incorrectly labelled as guest samples:

About the before kvm_before_interrupt() ...

> 
> # test: perf-record + cpu-cycles:HP (which collects host-only precise samples)
> # Symbol                                   Overhead       sys       usr  guest sys  guest usr
> # .......................................  ........  ........  ........  .........  .........
> #
> # Before:
>   [g] entry_SYSCALL_64                       24.63%     0.00%     0.00%     24.63%      0.00%
>   [g] syscall_return_via_sysret              23.23%     0.00%     0.00%     23.23%      0.00%
>   [g] files_lookup_fd_raw                     6.35%     0.00%     0.00%      6.35%      0.00%
> # After:
>   [k] perf_adjust_freq_unthr_context         57.23%    57.23%     0.00%      0.00%      0.00%
>   [k] __vmx_vcpu_run                          4.09%     4.09%     0.00%      0.00%      0.00%
>   [k] vmx_update_host_rsp                     3.17%     3.17%     0.00%      0.00%      0.00%
> 
> In the above case, perf records the samples labelled '[g]', the RIPs behind
> the weird samples are actually being queried by perf_instruction_pointer()
> after determining whether it's in GUEST state or not, and here's the issue:
> 
> If vm-exit is caused by a non-NMI interrupt (such as hrtimer_interrupt) and
> at least one PMU counter is enabled on host, the kvm_arch_pmi_in_guest()
> will remain true (KVM_HANDLING_IRQ is set) until kvm_before_interrupt().

... and here.

Would you mind helping why kvm_arch_pmi_in_guest() remains true before
*kvm_before_interrupt()*.

According to the source code, the vcpu->arch.handling_intr_from_guest
is set to non-zero only at kvm_before_interrupt(), and cleared at
kvm_after_interrupt().

Or would you mean kvm_after_interrupt()?

Thank you very much!

Dongli Zhang

> 
> During this window, if a PMI occurs on host (since the KVM instructions on
> host are being executed), the control flow, with the help of the host NMI
> context, will be transferred to perf/core to generate performance samples,
> thus perf_instruction_pointer() and perf_guest_get_ip() is called.
> 
> Since kvm_arch_pmi_in_guest() only checks if there is an interrupt, it may
> cause perf/core to mistakenly assume that the source RIP of the host NMI
> belongs to the guest world and use perf_guest_get_ip() to get the RIP of
> a vCPU that has already exited by a non-NMI interrupt.
> 
> Error samples are recorded and presented to the end-user via perf-report.
> Such false positive samples could be eliminated by explicitly determining
> if the exit reason is KVM_HANDLING_NMI.
> 
> Note that when vm-exit is indeed triggered by PMI and before HANDLING_NMI
> is cleared, it's also still possible that another PMI is generated on host.
> Also for perf/core timer mode, the false positives are still possible since
> that non-NMI sources of interrupts are not always being used by perf/core.
> In both cases above, perf/core should correctly distinguish between real
> RIP sources or even need to generate two samples, belonging to host and
> guest separately, but that's perf/core's story for interested warriors.
> 
> Fixes: dd60d217062f ("KVM: x86: Fix perf timer mode IP reporting")
> Signed-off-by: Like Xu <likexu@xxxxxxxxxxx>
> ---
> V1 -> V2 Changelog:
> - Refine commit message to cover both perf/core timer and NMI modes;
> - Use in_nmi() to distinguish whether it's NMI mode or not; (Sean)
> V1: https://urldefense.com/v3/__https://lore.kernel.org/kvm/20231204074535.9567-1-likexu@xxxxxxxxxxx/__;!!ACWV5N9M2RV99hQ!MQ8FetD27SVKN34CS_P-K3qrhspFnpf_Mqb0McFN9y5vSUeScc5b0TlZ3ZMDvt4Cn4b3g0h9ci6EO9k3PBEQXpePrg$ 
>  arch/x86/include/asm/kvm_host.h | 10 +++++++++-
>  arch/x86/kvm/x86.h              |  6 ------
>  2 files changed, 9 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index c8c7e2475a18..167d592e08d0 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -1868,8 +1868,16 @@ static inline int kvm_arch_flush_remote_tlbs_range(struct kvm *kvm, gfn_t gfn,
>  }
>  #endif /* CONFIG_HYPERV */
>  
> +enum kvm_intr_type {
> +	/* Values are arbitrary, but must be non-zero. */
> +	KVM_HANDLING_IRQ = 1,
> +	KVM_HANDLING_NMI,
> +};
> +
> +/* Enable perf NMI and timer modes to work, and minimise false positives. */
>  #define kvm_arch_pmi_in_guest(vcpu) \
> -	((vcpu) && (vcpu)->arch.handling_intr_from_guest)
> +	((vcpu) && (vcpu)->arch.handling_intr_from_guest && \
> +	 (in_nmi() == ((vcpu)->arch.handling_intr_from_guest == KVM_HANDLING_NMI)))
>  
>  void __init kvm_mmu_x86_module_init(void);
>  int kvm_mmu_vendor_module_init(void);
> diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
> index 2f7e19166658..4dc38092d599 100644
> --- a/arch/x86/kvm/x86.h
> +++ b/arch/x86/kvm/x86.h
> @@ -431,12 +431,6 @@ static inline bool kvm_notify_vmexit_enabled(struct kvm *kvm)
>  	return kvm->arch.notify_vmexit_flags & KVM_X86_NOTIFY_VMEXIT_ENABLED;
>  }
>  
> -enum kvm_intr_type {
> -	/* Values are arbitrary, but must be non-zero. */
> -	KVM_HANDLING_IRQ = 1,
> -	KVM_HANDLING_NMI,
> -};
> -
>  static __always_inline void kvm_before_interrupt(struct kvm_vcpu *vcpu,
>  						 enum kvm_intr_type intr)
>  {
> 
> base-commit: 1ab097653e4dd8d23272d028a61352c23486fd4a