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

[Xiaoyao Li]

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?