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

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Message-ID: <20231206032054.55070-1-likexu@tencent.com>
Date:   Wed,  6 Dec 2023 11:20:54 +0800
From:   Like Xu <like.xu.linux@...il.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Andi Kleen <ak@...ux.intel.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH v2] KVM: x86/intr: Explicitly check NMI from guest to eliminate false positives

From: Like Xu <likexu@...cent.com>

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:

# 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().

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@...cent.com>
---
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://lore.kernel.org/kvm/20231204074535.9567-1-likexu@tencent.com/
 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
-- 
2.43.0