Re: [PATCH v2 2/2] KVM: selftests: Add logic to detect if ioctl() failed because VM was killed

[Xiaoyao Li <xiaoyao.li@intel.com>]

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.