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

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Message-ID: <aJFUspObVxdqInBo@google.com>
Date: Mon, 4 Aug 2025 17:47:46 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: dan.j.williams@...el.com
Cc: Xu Yilun <yilun.xu@...ux.intel.com>, Chao Gao <chao.gao@...el.com>, 
	linux-coco@...ts.linux.dev, x86@...nel.org, kvm@...r.kernel.org, 
	pbonzini@...hat.com, eddie.dong@...el.com, kirill.shutemov@...el.com, 
	dave.hansen@...el.com, kai.huang@...el.com, isaku.yamahata@...el.com, 
	elena.reshetova@...el.com, rick.p.edgecombe@...el.com, 
	Farrah Chen <farrah.chen@...el.com>, 
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>, Dave Hansen <dave.hansen@...ux.intel.com>, 
	Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	"H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 07/20] x86/virt/tdx: Expose SEAMLDR information via sysfs

On Mon, Aug 04, 2025, dan.j.williams@...el.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.