On Thu, Nov 30, 2023, Xiaoyao Li wrote: > On 11/30/2023 3:22 AM, Sean Christopherson wrote: > > 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. > > If -EIO for KVM ioctl denotes "the VM is dead" is to be the officially > announced API, I'm fine. Yes, -EIO is effectively ABI at this point. Though there is the caveat that -EIO doesn't guarantee KVM killed the VM, i.e. KVM could return -EIO for some other reason (though that's highly unlikely for KVM_RUN at least). > > 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, > > I was trying to ask if KVM_EXIT_SHUTDOWN and KVM_EXIT_INTERNAL_ERROR are > treated as fatal error by userspace. Ah. Not really. SHUTDOWN isn't fatal per se, e.g. QEMU emulates a RESET if a vCPU hits shutdown. INTERNAL_ERROR isn't always fatal on x86, e.g. QEMU ignores (I think that's what happens) emulation failure when the vCPU is at CPL > 0 so that guest userspace can't DoS the VM.
