Re: [RFC PATCH 07/20] x86/virt/tdx: Expose SEAMLDR information via sysfs

[Sean Christopherson <seanjc@google.com>]

On Mon, Aug 04, 2025, dan.j.williams@intel.com wrote:
> Xu Yilun wrote:
> > So my idea is to remove tdx_tsm device (thus disables tdx_tsm driver) on
> > vmxoff.
> > 
> >   KVM                TDX core            TDX TSM driver
> >   -----------------------------------------------------
> >   tdx_disable()
> >                      tdx_tsm dev del
> >                                          driver.remove()
> >   vmxoff()
> > 
> > An alternative is to move vmxon/off management out of KVM, that requires
> > a lot of complex work IMHO, Chao & I both prefer not to touch it.

Eh, it's complex, but not _that_ complex.

> It is fine to require that vmxon/off management remain within KVM, and
> tie the lifetime of the device to the lifetime of the kvm_intel module*.

Nah, let's do this right.  Speaking from experience; horrible, make-your-eyes-bleed
experience; playing games with kvm-intel.ko to try to get and keep CPUs post-VMXON
will end in tears.

And it's not just TDX-feature-of-the-day that needs VMXON to be handled outside
of KVM, I'd also like to do so to allow out-of-tree hypervisors to do the "right
thing"[*].  Not because I care deeply about out-of-tree hypervisors, but because
the lack of proper infrastructure for utilizing virtualization hardware irks me.

The basic gist is to extract system-wide resources out of KVM and into a separate
module, so that e.g. tdx_tsm or whatever can take a dependency on _that_ module
and elevate refcounts as needed.  All things considered, there aren't so many
system-wide resources that it's an insurmountable task.

I can provide some rough patches to kickstart things.  It'll probably take me a
few weeks to extract them from an old internal branch, and I can't promise they'll
compile.  But they should be good enough to serve as an RFC.

https://lore.kernel.org/all/ZwQjUSOle6sWARsr@google.com

> * It would be unfortunate if userspace needed to manually probe for TDX
>   Connect when KVM is not built-in. We might add a simple module that
>   requests kvm_intel in that case:

Oh hell no :-)

We have internal code that "requests" vendor module, and it might just be my least
favorite thing.  Juggling the locks and module lifetimes is just /shudder.