Alexandru Elisei <alexandru.elisei@xxxxxxx> writes: > Hi, > > On Tue, Feb 11, 2025 at 01:08:51PM +0530, Aneesh Kumar K.V (Arm) wrote: >> The return value for the KVM_RUN ioctl is confusing and has led to >> errors in different kernel exit handlers. A return value of 0 indicates >> a return to the VMM, whereas a return value of 1 indicates resuming >> execution in the guest. Some handlers mistakenly return 0 to force a >> return to the guest. > > I find this paragraph confusing. KVM_RUN, as per the documentation, returns 0 or > -1 (on error). As far as I can tell, at least on arm64, KVM_RUN can never return > 1. > > Are you referring to the loop in kvm_arch_vcpu_ioctl_run() by any chance? That's > the only place I found where a value of 1 from the handlers signifies return to > the guest. > Yes. I will update the commit message to reflect that. It is the exit handler return value rather than KVM_RUN ioctl return value. > >> >> This worked in kvmtool because the exit_reason defaulted to >> 0 (KVM_EXIT_UNKNOWN), and kvmtool did not error out on an unknown exit >> reason. However, forcing a KVM panic on an unknown exit reason would >> help catch these bugs early. > > I would hope that a VMM cannot force KVM to panic at will, which will bring down > the host. Are you referring to kvmtool exiting with an error? That's what the > unfortunately named 'panic_kvm' label seems to be doing. > yes. I will update the commit message to indicate that for KVM_EXIT_UNKNOWN exit reason, kvmtool will now exit with an error instead of returning to the guest." -aneesh
