Messages in this thread | | | Date | Mon, 13 Nov 2023 12:04:39 +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 11/9/2023 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.
> Using an exit reason is a also bit tricky because it requires a vCPU, whereas a > dead VM blocks anything and everything.
No argue of it. It cannot work for all the case, but at least it can make some case happier.
>> 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.
what about KVM_EXIT_SHUTDOWN? Or KVM_EXIT_INTERNAL_ERROR?
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