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

[Jim Mattson]

On Tue, Apr 7, 2026 at 9:40 AM Pawan Gupta
<pawan.kumar.gupta@xxxxxxxxxxxxxxx> wrote:
>
> 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.

I don't think it's reasonable for the guest to rely on a future
implementation to enforce the appropriate protections.

This is already a problem today. If a guest sees that BHI_CTRL is
unavailable, it will deploy the short BHB clearing sequence and
declare that the vulnerability is mitigated. That isn't true if the
guest is running on Alder Lake or newer.