Re: [PATCH v9 02/10] x86/bhi: Make clear_bhb_loop() effective on newer CPUs

[Pawan Gupta]

On Mon, Apr 06, 2026 at 07:23:25AM -0700, Jim Mattson wrote:
> Yes, but the guest needs a way to determine whether the hypervisor
> will do what's necessary to make the short sequence effective. And, in
> particular, no KVM hypervisor today is prepared to do that.
> 
> When running under a hypervisor, without BHI_CTRL and without any
> evidence to the contrary, the guest must assume that the longer
> sequence is necessary. At the very least, we need a CPUID or MSR bit
> that says, "the short BHB clearing sequence is adequate for this
> vCPU."

After discussing this internally, the consensus is that the best path
forward is to add virtual SPEC_CTRL support to KVM, which also aligns with
Intel's guidance. In the long term, virtual SPEC_CTRL can benefit future
mitigations as well. As with many other mitigations (e.g. microcode), the
guest would rely on the host to enforce the appropriate protections.