linux-kernel - Re: [PATCH v19 035/130] KVM: TDX: Add place holder for TDX VM specific mem_enc

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Date: Tue, 26 Mar 2024 03:10:13 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "Huang, Kai" <kai.huang@...el.com>, "Yamahata, Isaku"
	<isaku.yamahata@...el.com>
CC: "Zhang, Tina" <tina.zhang@...el.com>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "Yuan, Hang" <hang.yuan@...el.com>,
	"seanjc@...gle.com" <seanjc@...gle.com>, "Chen, Bo2" <chen.bo@...el.com>,
	"sagis@...gle.com" <sagis@...gle.com>, "isaku.yamahata@...ux.intel.com"
	<isaku.yamahata@...ux.intel.com>, "Aktas, Erdem" <erdemaktas@...gle.com>,
	"isaku.yamahata@...il.com" <isaku.yamahata@...il.com>, "kvm@...r.kernel.org"
	<kvm@...r.kernel.org>, "pbonzini@...hat.com" <pbonzini@...hat.com>
Subject: Re: [PATCH v19 035/130] KVM: TDX: Add place holder for TDX VM
 specific mem_enc_op ioctl

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.