On Wed, Nov 08, 2023, Xiaoyao Li wrote: > On 11/8/2023 9:09 AM, Sean Christopherson wrote: > > Add yet another macro to the VM/vCPU ioctl() framework to detect when an > > ioctl() failed because KVM killed/bugged the VM, i.e. when there was > > nothing wrong with the ioctl() itself. If KVM kills a VM, e.g. by way of > > a failed KVM_BUG_ON(), all subsequent VM and vCPU ioctl()s will fail with > > -EIO, which can be quite misleading and ultimately waste user/developer > > time. > > > > Use KVM_CHECK_EXTENSION on KVM_CAP_USER_MEMORY to detect if the VM is > > dead and/or bug, as KVM doesn't provide a dedicated ioctl(). Using a > > heuristic is obviously less than ideal, but practically speaking the logic > > is bulletproof barring a KVM change, and any such change would arguably > > break userspace, e.g. if KVM returns something other than -EIO. > > We hit similar issue when testing TDX VMs. Most failure of SEMCALL is > handled with a KVM_BUG_ON(), which leads to vm dead. Then the following > IOCTL from userspace (QEMU) and gets -EIO. > > Can we return a new KVM_EXIT_VM_DEAD on KVM_REQ_VM_DEAD? Why? Even if KVM_EXIT_VM_DEAD somehow provided enough information to be useful from an automation perspective, the VM is obviously dead. I don't see how the VMM can do anything but log the error and tear down the VM. KVM_BUG_ON() comes with a WARN, which will be far more helpful for a human debugger, e.g. because all vCPUs would exit with KVM_EXIT_VM_DEAD, it wouldn't even identify which vCPU initially triggered the issue. Using an exit reason is a also bit tricky because it requires a vCPU, whereas a dead VM blocks anything and everything. > and replace -EIO with 0? yes, it's a ABI change. Definitely a "no" on this one. As has been established by the guest_memfd series, it's ok to return -1/errno with a valid exit_reason. > But I'm wondering if any userspace relies on -EIO behavior for VM DEAD case. I doubt userspace relies on -EIO, but userpsace definitely relies on -1/errno being returned when a fatal error.
