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Older VMX supporting CPUs do not provide the "Virtual NMI" feature for tracking the NMI-blocked state after injecting such events. For now KVM is unable to inject NMIs on those CPUs. Derived from Sheng Yang's suggestion to use the IRQ window notification for detecting the end of NMI handlers, this patch implements virtual NMI support without impact on the host's ability to receive real NMIs. The downside is that the given approach requires some heuristics that can cause NMI nesting in vary rare corner cases. The approach works as follows: - inject NMI and set a software-based NMI-blocked flag - arm the IRQ window start notification whenever an NMI window is requested - if the guest exits due to an opening IRQ window, clear the emulated NMI-blocked flag - if the guest net execution time with NMI-blocked but without an IRQ window exceeds 1 second, force NMI-blocked reset and inject anyway This approach covers most practical scenarios: - succeeding NMIs are seperated by at least one open IRQ window - the guest may spin with IRQs disabled (e.g. due to a bug), but leaving the NMI handler takes much less time than one second - the guest does not rely on strict ordering or timing of NMIs (would be problematic in virtualized environments anyway) Successfully tested with the 'nmi n' monitor command, the kgdbts testsuite on smp guests (additional patches required to add debug register support to kvm) + the kernel's nmi_watchdog=1, and a Siemens- specific board emulation (+ guest) that comes with its own NMI watchdog mechanism. Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com> Signed-off-by: Avi Kivity <avi@redhat.com> |
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