Messages in this thread | | | Date | Thu, 30 Nov 2023 11:04:08 +0800 | Subject | Re: [PATCH v2 2/2] KVM: selftests: Add logic to detect if ioctl() failed because VM was killed | From | Xiaoyao Li <> |
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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.
> 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.
> 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.
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