On Mon, Nov 13, 2023, Xiaoyao Li wrote: > On 11/9/2023 12:07 AM, Sean Christopherson wrote: > > 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. > > It's not about providing more helpful debugging info, but to provide a > dedicated notification for VMM that "the VM is dead, all the following > command may not response". With it, VMM can get rid of the tricky detection > like this patch. But a VMM doesn't need this tricky detection, because this tricky detections isn't about detecting that the VM is dead, it's all about helping a human debug why a test failed. -EIO already effectively says "the VM is dead", e.g. QEMU isn't going to keep trying to run vCPUs. Similarly, selftests assert either way, the goal is purely to print out a unique error message to minimize the chances of confusing the human running the test (or looking at results). > > 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. > > what about KVM_EXIT_SHUTDOWN? Or KVM_EXIT_INTERNAL_ERROR? I don't follow, those are vcpu_run.exit_reason values, not errno values. Returning any flavor of KVM_EXIT_*, which are positive values, would break userspace, e.g. QEMU explicitly looks for "ret < 0", and glibc only treats small-ish negative values as errors, i.e. a postive return value will be propagated verbatim up to QEMU.
