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:
# 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@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://lore.kernel.org/kvm/20231204074535.9567-1-likexu@xxxxxxxxxxx/
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
