Re: [PATCH v3 00/24] KVM: TDX huge page support for private memory

[Sean Christopherson <seanjc@google.com>]

On Tue, Feb 03, 2026, Yan Zhao wrote:
> On Fri, Jan 30, 2026 at 07:32:48AM -0800, Sean Christopherson wrote:
> > On Mon, Jan 19, 2026, Yan Zhao wrote:
> > > On Sat, Jan 17, 2026 at 12:58:02AM +0800, Edgecombe, Rick P wrote:
> > > > On Fri, 2026-01-16 at 08:31 -0800, Sean Christopherson wrote:
> > > IIUC, this concern should be gone as Dave has agreed to use "pfn" as the
> > > SEAMCALL parameter [1]?
> > > Then should we invoke "KVM_MMU_WARN_ON(!tdx_is_convertible_pfn(pfn));" in KVM
> > > for every pfn of a huge mapping? Or should we keep the sanity check inside the
> > > SEAMCALL wrappers?
> > 
> > I don't have a strong preference.  But if it goes in KVM, definitely guard it with
> > KVM_MMU_WARN_ON().
> Thank you for your insights, Sean!
> 
> > > BTW, I have another question about the SEAMCALL wrapper implementation, as Kai
> > > also pointed out in [2]: since the SEAMCALL wrappers now serve as APIs available
> > > to callers besides KVM, should the SEAMCALL wrappers return TDX_OPERAND_INVALID
> > > or WARN_ON() (or WARN_ON_ONCE()) on sanity check failure?
> > 
> > Why not both?  But maybe TDX_SW_ERROR instead of TDX_OPERAND_INVALID?
> Hmm, I previously returned TDX_OPERAND_INVALID for non-aligned base PFN.
> TDX_SW_ERROR is also ok if we want to indicate that passing an invalid PFN is a
> software error.
> (I had tdh_mem_page_demote() return TDX_SW_ERROR when an incompatible TDX module
> is used, i.e., when !tdx_supports_demote_nointerrupt()).
> 
> > If an API has a defined contract and/or set of expectations, and those expectations
> > aren't met by the caller, then a WARN is justified.  But the failure still needs
> > to be communicated to the caller.
> Ok.
> 
> The reason for 'not both' is that there's already TDX_BUG_ON_2() in KVM after
> the SEAMCALL wrapper returns a non-BUSY error. I'm not sure if having double
> WARN_ON_ONCE() calls is good, so I intended to let the caller decide whether to
> warn.

Two WARNs isn't the end of the world.  It might even be helpful in some cases,
e.g. to more precisely document what went wrong.

> > > By returning TDX_OPERAND_INVALID, the caller can check the return code, adjust
> > > the input or trigger WARN_ON() by itself;
> > > By triggering WARN_ON() directly in the SEAMCALL wrapper, we need to document
> > > this requirement for the SEAMCALL wrappers and have the caller invoke the API
> > > correctly.
> > 
> > Document what exactly?  Most of this should be common sense.  E.g. we don't generally
> > document that pointers must be non-NULL, because that goes without saying 99.9%
> > of the time.
> Document the SEAMCALL wrapper's expectations. e.g., for demote, a PFN must be
> 2MB-aligned, or the caller must not invoke tdh_mem_page_demote() if a TDX module
> does not support feature ENHANCED_DEMOTE_INTERRUPTIBILITY...

FWIW, for me, all of those are self-explanatory and/or effectively covered by the
TDX specs.

> > IMO, that holds true here as well.  E.g. trying to map memory into a TDX guest
> > that isn't convertible is obviously a bug, I don't see any value in formally
> > documenting that requirement.
> Do we need a comment for documentation above the tdh_mem_page_demote() API?

I wouldn't bother, but I truly don't care if the TDX subsystem wants to document
everything in gory detail.