Re: [PATCH] KVM: SVM: Add module parameter to control SEV-SNP Secure TSC feature

[Sean Christopherson <seanjc@google.com>]

On Wed, Oct 29, 2025, Tom Lendacky wrote:
> On 10/29/25 08:58, Sean Christopherson wrote:
> > On Wed, Oct 29, 2025, Nikunj A Dadhania wrote:
> >> Add a module parameter secure_tsc to allow control of the SEV-SNP Secure
> >> TSC feature at module load time, providing administrators with the ability
> >> to disable Secure TSC support even when the hardware and kernel support it.
> > 
> > Why?
> 
> That's on me. Based on the debug_swap parameter I thought we wanted to
> be able to control all SEV features that are advertised and thought this
> was just missed for Secure TSC. I'm good with not adding it we don't
> need to do that.

DebugSwap was one big mistake.  At this point, I think we can and should rip out
its module param.

Commit d1f85fbe836e ("KVM: SEV: Enable data breakpoints in SEV-ES") goofed by not
adding a way for the userspace VMM to control the feature.  Functionally, that was
fine, but it broke attestation signatures because SEV_FEATURES are included in the
signature.

Commit 5abf6dceb066 ("SEV: disable SEV-ES DebugSwap by default") fixed that issue,
but the underlying flaw of userspace not having a way to control SEV_FEATURES was
still there.

That flaw was addressed by commit 4f5defae7089 ("KVM: SEV: introduce KVM_SEV_INIT2
operation"), and so then 4dd5ecacb9a4 ("KVM: SEV: allow SEV-ES DebugSwap again")
re-enabled DebugSwap by default.

Now that the dust is settled, the module param doesn't serve any meaningful purpose.