Re: [PATCH v3 02/24] x86/virt/tdx: Add SEAMCALL wrapper tdh_mem_page_demote()

["Huang, Kai" <kai.huang@intel.com>]

On Fri, 2026-01-16 at 16:35 +0800, Yan Zhao wrote:
> Hi Kai,
> Thanks for reviewing!
> 
> On Fri, Jan 16, 2026 at 09:00:29AM +0800, Huang, Kai wrote:
> > 
> > > 
> > > Enable tdh_mem_page_demote() only on TDX modules that support feature
> > > TDX_FEATURES0.ENHANCE_DEMOTE_INTERRUPTIBILITY, which does not return error
> > > TDX_INTERRUPTED_RESTARTABLE on basic TDX (i.e., without TD partition) [2].
> > > 
> > > This is because error TDX_INTERRUPTED_RESTARTABLE is difficult to handle.
> > > The TDX module provides no guaranteed maximum retry count to ensure forward
> > > progress of the demotion. Interrupt storms could then result in a DoS if
> > > host simply retries endlessly for TDX_INTERRUPTED_RESTARTABLE. Disabling
> > > interrupts before invoking the SEAMCALL also doesn't work because NMIs can
> > > also trigger TDX_INTERRUPTED_RESTARTABLE. Therefore, the tradeoff for basic
> > > TDX is to disable the TDX_INTERRUPTED_RESTARTABLE error given the
> > > reasonable execution time for demotion. [1]
> > > 
> > 
> > [...]
> > 
> > > v3:
> > > - Use a var name that clearly tell that the page is used as a page table
> > >   page. (Binbin).
> > > - Check if TDX module supports feature ENHANCE_DEMOTE_INTERRUPTIBILITY.
> > >   (Kai).
> > > 
> > [...]
> > 
> > > +u64 tdh_mem_page_demote(struct tdx_td *td, u64 gpa, int level, struct page *new_sept_page,
> > > +			u64 *ext_err1, u64 *ext_err2)
> > > +{
> > > +	struct tdx_module_args args = {
> > > +		.rcx = gpa | level,
> > > +		.rdx = tdx_tdr_pa(td),
> > > +		.r8 = page_to_phys(new_sept_page),
> > > +	};
> > > +	u64 ret;
> > > +
> > > +	if (!tdx_supports_demote_nointerrupt(&tdx_sysinfo))
> > > +		return TDX_SW_ERROR;
> > > 
> > 
> > For the record, while I replied my suggestion [*] to this patch in v2, it
> > was basically because the discussion was already in that patch -- I didn't
> > mean to do this check inside tdh_mem_page_demote(), but do this check in
> > KVM page fault patch and return 4K as maximum mapping level.
> > 
> > The precise words were:
> > 
> >   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?
> Right. I followed it in the last patch (patch 24).
> 
> > Looking at this series, this is eventually done in your last patch.  But I
> > don't quite understand what's the additional value of doing such check and
> > return TDX_SW_ERROR in this SEAMCALL wrapper.
> > 
> > Currently in this series, it doesn't matter whether this wrapper returns
> > TDX_SW_ERROR or the real TDX_INTERRUPTED_RESTARTABLE -- KVM terminates the
> > TD anyway (see your patch 8) because this is unexpected as checked in your
> > last patch.
> > 
> > IMHO we should get rid of this check in this low level wrapper.
> You are right, the wrapper shouldn't hit this error after the last patch.
> 
> However, I found it's better to introduce the feature bit
> TDX_FEATURES0_ENHANCE_DEMOTE_INTERRUPTIBILITY and the helper
> tdx_supports_demote_nointerrupt() together with the demote SEAMCALL wrapper.
> This way, people can understand how the TDX_INTERRUPTED_RESTARTABLE error is
> handled for this SEAMCALL. 
> 

So the "handling" here is basically making DEMOTE SEAMCALL unavailable
when DEMOTE is interruptible at low SEAMCALL wrapper level.

I guess you can argue this has some value since it tells users "don't even
try to call me when I am interruptible because I am not available".  

However, IMHO this also implies the benefit is mostly for the case where
the user wants to use this wrapper to tell whether DEMOTE is available. 
E.g.,

	err = tdh_mem_page_demote(...);
	if (err == TDX_SW_ERROR)
		enable_tdx_hugepage = false;

But in this series you are using tdx_supports_demote_nointerrupt() for
this purpose, which is better IMHO.

So maybe there's a *theoretical* value to have the check here, but I don't
see any *real* value.

But I don't have strong opinion either -- I guess I just don't like making
these low level SEAMCALL wrappers more complicated than what the SEAMCALL
does -- and it's up to you to decide. :-)

> 
> What do you think about changing it to a WARN_ON_ONCE()? i.e.,
> WARN_ON_ONCE(!tdx_supports_demote_nointerrupt(&tdx_sysinfo));

What's your intention?

W/o the WARN(), the caller _can_ call this wrapper (i.e., not a kernel
bug) but it always get a SW-defined error.  Again, maybe it has value for
the case where the caller wants to use this to tell whether DEMOTE is
available.

With the WARN(), it's a kernel bug to call the wrapper, and the caller
needs to use other way (i.e., tdx_supports_demote_nointerrupt()) to tell
whether DEMOTE is available.

So if you want the check, probably WARN() is a better idea since I suppose
we always want users to use tdx_supports_demote_nointerrupt() to know
whether DEMOTE can be done, and the WARN() is just to catch bug.