Re: [PATCH v19 035/130] KVM: TDX: Add place holder for TDX VM specific mem_enc_op ioctl

[Edgecombe, Rick P]

On Sat, 2024-03-23 at 04:27 +0000, Huang, Kai wrote:
> > vt_vcpu_mem_enc_ioctl() checks non-TDX case and returns -ENOTTY.  We know that
> > the guest is TD.
> 
> But the command is not supported, right?
> 
> I roughly recall I saw somewhere that in such case we should return -ENOTTY, but
> I cannot find the link now.
> 
> But I found this old link uses -ENOTTY:
> 
> https://lwn.net/Articles/58719/
> 
> So, just fyi.

The AMD version of this returns -EINVAL when the subcommand is not implemented. I don't think the
TDX side should need to necessarily match that. Is the case of concern when in a future where there
are more subcommands that are only supported when some other mode is enabled?

The man page says:
       ENOTTY The specified request does not apply to the kind of object
              that the file descriptor fd references.

If a future command does not apply for the TDX mode, then an upgraded kernel could start returning
ENOTTY instead of EINVAL. Hmm. We could always have the option of making KVM_MEMORY_ENCRYPT_OP_FOO
for some future mode foo if there were compatibility issues, so I don't think we would be stuck
either way. 

After thinking about it, I'd make a weak vote to leave it. No strong opinion though.