Re: [RFC PATCH v2 02/23] x86/virt/tdx: Add SEAMCALL wrapper tdh_mem_page_demote()

[Yan Zhao]

On Tue, Nov 11, 2025 at 05:15:22PM +0800, Huang, Kai wrote:
> On Mon, 2025-09-01 at 17:08 +0800, Yan Zhao wrote:
> > > > Do not handle TDX_INTERRUPTED_RESTARTABLE because SEAMCALL
> > > > TDH_MEM_PAGE_DEMOTE does not check interrupts (including NMIs) for basic
> > > > TDX (with or without Dynamic PAMT).
> > > 
> > > The cover letter mentions that there is a new TDX module in planning, which
> > > disables the interrupt checking. I guess TDX module would need to have a
> > > interface to report the change, KVM then decides to enable huge page support or
> > > not for TDs?
> > Yes. But I guess detecting TDX module version or if it supports certain feature
> > is a generic problem. e.g., certain versions of TDX module have bugs in
> > zero-step mitigation and may block vCPU entering.
> > 
> > So, maybe it deserves a separate series?
> 
> Looking at the spec (TDX module ABI spec 348551-007US), is it enumerated via
> TDX_FEATURES0.ENHANCED_DEMOTE_INTERRUPTIBILITY?
Yes. I checked the unreleased TDX module code that enumerates this bit (starting
from version TDX_1.5.28.00.972). TDH.MEM.PAGE.DEMOTE will not return
TDX_INTERRUPTED_RESTARTABLE for L1 VMs.

>   5.4.25.3.9.
> 
>   Interruptibility
> 
>   If the TD is not partitioned (i.e., it has been configured with no L2 
>   VMs), and the TDX Module enumerates 
>   TDX_FEATURES0.ENHANCED_DEMOTE_INTERRUPTIBILITY as 1, TDH.MEM.PAGE.DEMOTE 
>   is not interruptible.
> 
> So if the decision is to not use 2M page when TDH_MEM_PAGE_DEMOTE can return
> TDX_INTERRUPTED_RESTARTABLE, maybe we can just check this enumeration in
> fault handler and always make mapping level as 4K?
Thanks for this info! I think this is a very good idea and the right direction.
If no objection, I'll update the code in this way.