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

["Edgecombe, Rick P" <rick.p.edgecombe@intel.com>]

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.

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

> 
> > But I don't think we do for security.

But, actually they are some of the same paths. So same pattern.

> > 
> > Speaking for Yan here, I think she was a little more worried about this scenario
> > then me, so I read this verbiage and thought to try to close it out.