On Tue, May 28, 2024, Paolo Bonzini wrote: > On Tue, May 28, 2024 at 6:19 AM Manali Shukla <manali.shukla@xxxxxxx> wrote: > > > > The upcoming new Idle HLT Intercept feature allows for the HLT > > instruction execution by a vCPU to be intercepted by the hypervisor > > only if there are no pending V_INTR and V_NMI events for the vCPU. > > When the vCPU is expected to service the pending V_INTR and V_NMI > > events, the Idle HLT intercept won’t trigger. The feature allows the > > hypervisor to determine if the vCPU is actually idle and reduces > > wasteful VMEXITs. > > Does this have an effect on the number of vmexits for KVM, unless AVIC > is enabled? Can you write a testcase for kvm-unit-tests' vmexit.flat > that shows an improvement? > > The reason I am wondering is because KVM does not really use V_INTR > injection. The "idle HLT" intercept basically differs from the basic > HLT trigger only in how it handles an STI;HLT sequence, as in that > case the interrupt can be injected directly and the HLT vmexit is > suppressed. But in that circumstance KVM would anyway use a V_INTR > intercept to detect the opening of the interrupt injection window (and > then the interrupt uses event injection rather than V_INTR). Again, > this is only true if AVIC is disabled, but that is the default. > > So unless I'm wrong in my analysis above, I'm not sure this series, > albeit small, is really worth it. But aren't we hoping to enable x2AVIC by default sooner than later? > As things stand, it would be more interesting to enable this for nested VMs, > especially Hyper-V which does use V_INTR and V_TPL; even better, _emulating_ > it on older processors would reduce the L2->L0->L1->L0->L2 path to a > less-expensive L2->L0->L2 vmexit.
