Re: [PATCH v2 16/17] KVM: TDX: Add in-kernel Quote generation

[Sean Christopherson <seanjc@google.com>]

On Mon, Jun 29, 2026, Peter Fang wrote:
> On Thu, Jun 25, 2026 at 11:01:58AM -0700, Sean Christopherson wrote:
> > On Thu, Jun 18, 2026, Xu Yilun wrote:
> > > From: Peter Fang <peter.fang@intel.com>
> > > 
> > > Provide an in-kernel path for Quote generation when handling
> > > TDG.VP.VMCALL<GetQuote>, without requiring an exit to userspace.
> > 
> > Why?
> > 
> 
> Hi Sean,
> 
> This is mainly to avoid a round trip to userspace for the GetQuote flow.

Again, why?

> New TDX modules can now get a Quote directly via an "extension SEAMCALL"
> instead of exiting to userspace and using an SGX enclave. Exiting to
> userspace for GetQuote no longer seems worth the overhead/complexity.

I dunno, from a kernel perspective, this is more complexity, not less:

 Documentation/arch/x86/tdx.rst |  19 ++---
 Documentation/virt/kvm/api.rst |   3 +
 arch/x86/include/asm/tdx.h     |   9 +++
 arch/x86/kvm/vmx/tdx.h         |   6 ++
 arch/x86/kvm/vmx/tdx.c         | 135 ++++++++++++++++++++++++++++++++-
 virt/kvm/kvm_main.c            |   1 +
 6 files changed, 163 insertions(+), 10 deletions(-)

> The first half of the series enables extension SEAMCALLs. They implement
> simple APIs for higher-order security protocols that would otherwise need
> to be broken into smaller routines. For Quoting, this allows KVM to get
> a Quote directly through TDH.QUOTE.GET. The TDX module needs only the
> input data from TDG.VP.VMCALL<GetQuote> for that call.

Answering my own question (though probably poorly), IIUC the answer is that
DICE-based quoting is done through the TDX Module, whereas existing quoting is
done through an SGX enclave and so was routed through userspace.

If that's all there is too this, then why is KVM involved?  I.e. why doesn't the
TDX Module provide the quote directly to the guest?