+KVM and LKML https://people.kernel.org/tglx/notes-about-netiquette On Mon, Oct 30, 2023, Prasad Pandit wrote: > Hello Sean, > > Please see: > -> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=bdedff263132c862924f5cad96f0e82eeeb4e2e6 > > * While testing a real-time host/guest setup, the above patch is > causing a strange regression wherien guest boot delays by indefinite > time. Sometimes it boots within a minute, sometimes it takes much > longer. Maybe the guest VM is waiting for a NMI event. > > * Reverting the above patch helps to fix this issue. I'm wondering if > a fix patch like below would be acceptable OR reverting above patch is > more reasonable? No, a revert would break AMD's vNMI. > === > # cat ~test/rpmbuild/SOURCES/linux-kernel-test.patch > +++ linux-5.14.0-372.el9/arch/x86/kvm/x86.c 2023-10-30 > 09:05:05.172815973 -0400 > @@ -5277,7 +5277,8 @@ static int kvm_vcpu_ioctl_x86_set_vcpu_e > if (events->flags & KVM_VCPUEVENT_VALID_NMI_PENDING) { > vcpu->arch.nmi_pending = 0; > atomic_set(&vcpu->arch.nmi_queued, events->nmi.pending); > - kvm_make_request(KVM_REQ_NMI, vcpu); > + if (events->nmi.pending) > + kvm_make_request(KVM_REQ_NMI, vcpu); This looks sane, but it should be unnecessary as KVM_REQ_NMI nmi_queued=0 should be a (costly) nop. Hrm, unless the vCPU is in HLT, in which case KVM will treat a spurious KVM_REQ_NMI as a wake event. When I made this change, my assumption was that userspace would set KVM_VCPUEVENT_VALID_NMI_PENDING iff there was relevant information to process. But if I'm reading the code correctly, QEMU invokes KVM_SET_VCPU_EVENTS with KVM_VCPUEVENT_VALID_NMI_PENDING at the end of machine creation. Hmm, but even that should be benign unless userspace is stuffing other guest state. E.g. KVM will spuriously exit to userspace with -EAGAIN while the vCPU is in KVM_MP_STATE_UNINITIALIZED, and I don't see a way for the vCPU to be put into a blocking state after transitioning out of UNINITIATED via INIT+SIPI without processing KVM_REQ_NMI. > } > static_call(kvm_x86_set_nmi_mask)(vcpu, events->nmi.masked); > === > > * Could you please have a look and suggest what could be a better fix? Please provide more information on what is breaking and/or how to reproduce the issue. E.g. at the very least, a trace of KVM_{G,S}ET_VCPU_EVENTS. There's not even enough info here to write a changelog.
