Re: [RFC PATCH v2 05/18] KVM: TDX: Drop superfluous page pinning in S-EPT management

[Sean Christopherson <seanjc@google.com>]

On Fri, Aug 29, 2025, Rick P Edgecombe wrote:
> On Fri, 2025-08-29 at 15:02 -0700, Sean Christopherson wrote:
> > On Fri, Aug 29, 2025, Rick P Edgecombe wrote:
> > > On Fri, 2025-08-29 at 13:19 -0700, Sean Christopherson wrote:
> > > > I'm happy to include more context in the changelog, but I really don't want
> > > > anyone to walk away from this thinking that pinning pages in random KVM code
> > > > is at all encouraged.
> > > 
> > > Sorry for going on a tangent. Defensive programming inside the kernel is a
> > > little more settled. But for defensive programming against the TDX module, there
> > > are various schools of thought internally. Currently we rely on some
> > > undocumented behavior of the TDX module (as in not in the spec) for correctness.
> > 
> > Examples?
> 
> I was thinking about the BUSY error code avoidance logic that is now called
> tdh_do_no_vcpus(). We assume no new conditions will appear that cause a
> TDX_OPERAND_BUSY. Like a guest opt-in or something.

Ah, gotcha.  If that happens, that's a TDX-Module ABI break.  Probably a good
idea to drill it into the TDX-Module authors/designers that ABI is established
when behavior is visible to the user, regardless of whether or not that behavior
is formally defined.

Note, breaking ABI _can_ be fine, e.g. if the behavior of some SEAMCALL changes,
but KVM doesn't care.  But if the TDX-Module suddenly starts failing a SEAMCALL
that previously succeeded, then we're going to have a problem.

> It's on our todo list to transition those assumptions to promises. We just need
> to formalize them.