Re: [PATCH v2 3/5] x86/virt/tdx: Add SEAMCALL wrapper for TDH.SYS.DISABLE

Kiryl Shutsemau
2026-04-01T09:26:43Z

On Tue, Mar 31, 2026 at 09:36:03PM +0000, Edgecombe, Rick P wrote:
> On Tue, 2026-03-31 at 18:22 +0000, Verma, Vishal L wrote:
> > >
> > > I guess the actual behaviour is dependant on the return code. It is
> > > obviously going to be the case for TDX_SUCCESS. And from the discussion,
> > > I guess that's true for TDX_SYS_BUSY and TDX_INTERRUPTED_RESUMABLE.
> > >
> > > What about other cases? The spec draft also lists TDX_SYS_NOT_READY and
> > > TDX_SYS_SHUTDOWN.
> >
> > I think these are safe too - TDX_SYS_SHUTDOWN means the module has
> > already been shutdown, which this seamcall would've done, so things
> > should be in the same state either way.
> >
> > TDX_SYS_NOT_READY means the module hasn't been initialized yet. This
> > seamcall should just exit, and the module is already blocking any
> > seamcall that need the module to be initialized. The seamcalls to
> > initialize the module will be allowed, as they are after a sys_disable
> > call anyway.
>
> Should the seamcall return success in the case where it would return
> TDX_SYS_NOT_READY? It is in basically a reset state right? The errors we care
> about are actual errors (TDX_SW_ERROR), so it makes no difference to the code in
> the patch. But it might be a nicer API for the seamcall?

I am not sure. TDX_SYS_NOT_READY can be useful as might indicate
mismatch of system state understanding between kernel and TDX module.

> > > I wounder if it can affect the kernel. Consider the case when kexec
> > > (crash kernel start) happens due to crash on TDX module.
> > >
> > > Will we be able to shutdown TDX module cleanly and make kexec safe?
> >
> > Hm  -are the semantics for what happens if there is a crash in the
> > module defined?

I meant kernel crash around/before TDX module initialization. Sorry for
confusion.

> > I think Linux should expect that sys_disable should
> > either start doing its shutdown work, or exit with one of the other
> > defined exit statuses. Anything else would be considered a module bug.
>
> We often have the question come up about how much we should to guard against
> bugs in the TDX module. I tend to also think we should not do defensive
> programming, same as we do for the kernel. If it's easy to handle something or
> emit a warning it's nice, but otherwise the solution for such cases should be to
> fix the TDX module bug.
>
> But for the kdump case, we don't actually need sys disable to succeed. The kdump
> kernel will not load the TDX module.

AFAIK, it is possible to start a normal kernel after kdump is done with
kexec (requires memmap= tricks). And the normal kernel might want to use
TDX again.

Not sure if it is done in practice. I would rather go full reboot path
after crash.

> And as for the errata, this already needs a
> special situation to be a problem. But even if it happens, I'd think better to
> try to the kdump. Not sure what the fix would be for that scenario, even if we
> allowed for a large complexity budget. So best effort seems good.
>
> Does it seem reasonable?

I am probably too picky here. We want to start from make basic kexec
functionality to work for start.

Reviewed-by: Kiryl Shutsemau (Meta) <kas@kernel.org>

--
Kiryl Shutsemau / Kirill A. Shutemov